Question: Arthur Gerald Wellesley, Earl of Mornington, is the great-great-grandson of what  last German Emperor ("Kaiser") and King of Prussia?
Context: Prince Wilhelm Karl of Prussia ("Wilhelm Karl Adalbert Erich Detloff"; 30 January 1922, in Potsdam – 9 April 2007, in Holzminden) was the third son of Prince Oskar of Prussia, and the last surviving grandson of Wilhelm II, the last German Emperor.  He was the thirty-sixth Master of Knights ("Herrenmeister") of the ancient and beneficent Protestant (and largely German) Order of Saint John (Bailiwick of Brandenburg), also known as "Der Johanniterorden". Wilhelm II or William II (German: "Friedrich Wilhelm Viktor Albert von Preußen", English: Frederick William Victor Albert of Prussia; 27 January 18594 June 1941) was the last German Emperor ("Kaiser") and King of Prussia, ruling the German Empire and the Kingdom of Prussia from 15 June 1888 to 9 November 1918.  He was the eldest grandchild of the British Queen Victoria and related to many monarchs and princes of Europe. Earl Cowley is a title in the Peerage of the United Kingdom.  It was created in 1857 for the diplomat Henry Wellesley, 2nd Baron Cowley.  He was Ambassador to France from 1852 to 1867.  He was made Viscount Dangan, of Dangan in the County of Meath, at the same time as he was given the earldom.  This title is also in the Peerage of the United Kingdom.  Lord Cowley was the eldest son of Henry Wellesley, 1st Baron Cowley, who like his son served as Ambassador to France.  In 1828 he was created Baron Cowley, of Wellesley in the County of Somerset, in the Peerage of the United Kingdom.  A member of the prominent Wellesley family, Cowley was the fifth and youngest son of Garret Wellesley, 1st Earl of Mornington, and the younger brother of Arthur Wellesley, 1st Duke of Wellington, and Richard Wellesley, 1st Marquess Wellesley. The monarchs of Prussia were members of the House of Hohenzollern who were the hereditary rulers of the former German state of Prussia from its founding in 1525 as the Duchy of Prussia.  The Duchy had evolved out of the Teutonic Order, a Roman Catholic crusader state and theocracy located along the eastern coast of the Baltic Sea.  The Teutonic Knights were under the leadership of a Grand Master, the last of whom, Albert, converted to Protestantism and secularized the lands, which then became the Duchy of Prussia.  The Duchy was initially a vassal of the Kingdom of Poland, as a result of the terms of the Prussian Homage whereby Albert was granted the Duchy as part of the terms of peace following the Prussian War.  When the main line of Prussian Hohenzollerns died out in 1618, the Duchy passed to a different branch of the family, who also reigned as Electors of Brandenburg in the Holy Roman Empire.  While still nominally two different territories, Prussia under the suzerainty of Poland and Brandenburg under the suzerainty of the Holy Roman Empire, the two states are known together historiographically as Brandenburg-Prussia.  Following the Second Northern War, a series of treaties freed the Duchy of Prussia from any vassalage to any other state, making it a fully sovereign Duchy in its own right.  This complex situation (where the Hohenzollern ruler of the independent Duchy of Prussia was also a subject of the Holy Roman Emperor as Elector of Brandenburg) laid the eventual groundwork for the establishment of the Kingdom of Prussia in 1701.  For diplomatic reasons, the rulers of the state were known as the King in Prussia from 1701 to 1772; largely because they still owed fealty to the Emperor as Electors of Brandenburg, the "King in Prussia" title (as opposed to "King of Prussia") avoided offending the Emperor.  As the Prussian state grew through several wars and diplomatic moves throughout the 18th century, it became apparent that Prussia had become a Great Power that did not need to submit meekly to the Holy Roman Empire.  By 1772, the pretense was dropped, and the style "King of Prussia" was adopted.  Thus it remained until 1871, when in the aftermath of the Franco-Prussian War, the King of Prussia Wilhelm I was crowned German Emperor.  From that point forward, though the Kingdom of Prussia retained its status as a constituent state of the German Empire, all remaining Kings of Prussia also served as German Emperor, and that title took precedence. The German Emperor (German: Deutscher Kaiser) was the official title of the head of state and hereditary ruler of the German Empire.  A specifically chosen term, it was used between the 18 January 1871 proclamation of King of Prussia and President of the North German Confederation Wilhelm I as ""Deutscher Kaiser"" and the official abdication of Wilhelm II on 28 November 1918.  The Holy Roman Emperor is sometimes also called "German Emperor" when the historical context is clear, as derived from the Holy Roman Empire's official post-1512 title of "Holy Roman Empire of the German Nation." Arthur Gerald Wellesley, Earl of Mornington (born 31 January 1978 at St Mary's Hospital in Paddington, London) is the son of Charles Wellesley, 9th Duke of Wellington.  The Wellesleys are an aristocratic Anglo-Irish family.  Through his mother, Antonia von Preussen, Duchess of Wellington, he is great-great-grandson of Kaiser Wilhelm II.  As a descendant of Queen Victoria, he is eligible to succeed to the British throne. Kaiser Wilhelm II abdicated as German Emperor and King of Prussia on 28 November 1918 from exile in Amerongen.  He had ruled Germany and Prussia from 15 June 1888 until 9 November 1918, when he went into exile.  Following the abdication statement and German Revolution of 1918–19, the German nobility as a legally defined class was abolished.  On promulgation of the Weimar Constitution on 11 August 1919, all Germans were declared equal before the law.  Altogether abolished were titles borne exclusively by monarchs; e.g., emperor/empress, king/queen, grand duke/grand duchess, etc.  There were 22 federal princes of the Kaiserreich ("within Germany"), who lost their titles and domains.  Of these princely heads of state, 4 held the title King (König) (the Kings of Prussia, Bavaria, Saxony, and Württemberg), 6 held the title Grand Duke (Großherzog), 5 held the title Duke (Herzog), and 7 held the title Prince (i.e. Sovereign Prince, "Fürst"). Gerald Wellesley, 7th Duke of Wellington, (21 August 1885 – 4 January 1972), styled Lord Gerald Wellesley between 1900 and 1943, was an Anglo-Irish diplomat, soldier, and architect. Georg Friedrich Ferdinand, Prince of Prussia (German: "Georg Friedrich Ferdinand Prinz von Preußen"; born 10 June 1976), is the current head of the House of Hohenzollern, the former ruling dynasty of the German Empire and of the Kingdom of Prussia.  He is the great-great-grandson and historic heir of Wilhelm II, the last German Emperor and King of Prussia, who was deposed and went into exile upon Germany's defeat in World War I in 1918. Frederick III (German: "Friedrich" ; 18 October 1831 – 15 June 1888) was German Emperor and King of Prussia for ninety-nine days in 1888, the Year of the Three Emperors.  Friedrich Wilhelm Nikolaus Karl, known informally as "Fritz", was the only son of Emperor Wilhelm I and was raised in his family's tradition of military service.  Although celebrated as a young man for his leadership and successes during the Second Schleswig, Austro-Prussian and Franco-Prussian wars, he nevertheless professed a hatred of warfare and was praised by friends and enemies alike for his humane conduct.  Following the unification of Germany in 1871 his father, then King of Prussia, became the German Emperor.  Upon Wilhelm's death at the age of ninety on 9 March 1888, the thrones passed to Frederick, who had by then been German Crown Prince for seventeen years and Crown Prince of Prussia for twenty-seven years.  Frederick was suffering from cancer of the larynx when he died on 15 June 1888, aged fifty-six, following unsuccessful medical treatments for his condition.
Answer: Wilhelm II