Question: The great-great-grandfather of current head of the House of Hohenzollern held what official title?

Context paragraphs:
Wilhelm II, German Emperor: 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.

List of mayors and lord mayors of Newcastle: This is a list of the Mayors and Lord Mayors of Newcastle City Council and its predecessors, a local government area of New South Wales, Australia.  The official title of Lord Mayors while holding office is: The Right Worshipful Lord Mayor of Newcastle.  First incorporated on 7 June 1859 as the 'Municipality of Newcastle', the council became known as 'The Borough of Newcastle' on 23 December 1867 following the enactment of The Municipalities Act of 1867, and on 1 April 1938 the 'City of Greater Newcastle' was proclaimed.  In recognition of Newcastle's role as NSW's second oldest and largest city, the council applied to have the title 'Lord Mayor', which was granted in October 1947 by King George VI and applied in October 1948.  This made Newcastle the first Australian city that was not a capital to receive such an honour.  On 1 April 1949 the official title of the council became the 'City of Newcastle'.

Iehiro Tokugawa: Iehiro Tokugawa (徳川家広 and 德川家廣 , Tokugawa Iehiro ) (born February 7, 1965) is an author, translator, and the heir of the main Tokugawa house.  Members of his Tokugawa clan ruled Japan as Shoguns from 1603 to 1867.  His great-great-grandfather was the famed Matsudaira Katamori of Aizu, and his maternal great-great-grandfather was Tokugawa Iesato, the sixteenth head of the Tokugawa clan.

William I, German Emperor: William I, or in German Wilhelm I (full name: "William Frederick Louis of Hohenzollern", German: "Wilhelm Friedrich Ludwig von Hohenzollern" , 22 March 1797 – 9 March 1888), of the House of Hohenzollern was King of Prussia from 2 January 1861 and the first German Emperor from 1 January 1871 to his death, the first Head of State of a united Germany.  Under the leadership of William and his Minister President Otto von Bismarck, Prussia achieved the unification of Germany and the establishment of the German Empire.  Despite his long support of Otto von Bismarck as Minister President, William held strong reservations about some of Bismarck's more reactionary policies, including his anti-Catholicism and tough handling of subordinates.  Contrary to the domineering Bismarck, William was described as polite, gentlemanly and, while a staunch conservative, more open to certain classical liberal ideas than his grandson Wilhelm II.

Prince Radu of Romania: Prince Radu of Romania (born Radu Duda 7 June 1960) is the son-in-law of former King Michael I of Romania.  He was born in Iași, Romania.  In 1996, he married Crown Princess Margareta of Romania, the King's eldest daughter.  On 1 January 1999, he was given the name, not title of "Prince of Hohenzollern-Veringen" by Friedrich Wilhelm, Prince of Hohenzollern, the Head of the Sigmaringen branch of the Hohenzollern family.  He has also called himself "Radu Hohenzollern Veringen Duda".  Since 2007, when he had his legal name changed from "Radu Duda" to "Radu al României Duda", Radu no longer uses the name of Hohenzollern.

Irina Mikhailovna Raievskya: Irina Mikhailovna Raievskya (18 August 1892 - 22 January 1955), (Russian: Раевская, Ирина Михайловна ), was Duchess of Mecklenburg, her second husband George, Duke of Mecklenburg (German: "Georg Herzog zu Mecklenburg" ) was the head of the House of Mecklenburg-Strelitz from 1934 until his death 1963.  Irina is the great-grandmother of Sophie, Princess of Prussia, wife of Georg Friedrich, Prince of Prussia current head of the House of Hohenzollern.

Georg Friedrich, Prince of Prussia: 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.

Countess Donata of Castell-Rüdenhausen: Countess Donata of Castell-Rüdenhausen (German: "Donata Gräfin zu Castell-Rüdenhausen"; 20 June 1950 – 5 September 2015) was a German noblewoman.  She was the daughter of (1916–2007), head of the Rüdenhausen branch of the House of Castell, and his wife, Countess Irene of Solms-Laubach (1925–2006).  She was the mother of Prince Georg Friedrich of Prussia (born 1976) the current head of the House of Hohenzollern, the ruling dynasty of the German Empire and of the Kingdom of Prussia until 1918.

Princess Herzeleide of Prussia: Princess Herzeleide-Ina-Marie Sophie Charlotte Else of Prussia (25 December 1918 – 22 March 1989) was a member of the deposed House of Hohenzollern.  She was the only daughter of Prince Oskar of Prussia (the second youngest son of Emperor Wilhelm II) and his morganatic wife ,Countess Ina Marie von Bassewitz.  She is the mother of Ernst-Johann Biron, Prince of Courland, the current head of the House of Biron.

GCE Ordinary Level: The O Level (Ordinary Level; official title: General Certificate of Education: Ordinary Level) is a subject-based qualification conferred as part of the General Certificate of Education.  It was introduced as part of British educational reform alongside the more in-depth and academically rigorous A-level (official title of qualification: General Certificate of Education – Advanced Level) in England, Wales and Northern Ireland.  Those three jurisdictions replaced O Levels gradually with General Certificate of Secondary Education (GCSE) and International General Certificate of Secondary Education (IGCSE) exams over time.  The Scottish equivalent was the O-grade (replaced by the Standard Grade).  The O Level qualification is still awarded by CIE Cambridge International Examinations, the international counterpart of the British examination Board OCR (Oxford, Cambridge & Royal Society of Arts), in select locations, instead of or alongside the International General Certificate of Secondary Education qualifications.  Both CIE and OCR have Cambridge Assessment as their parent organisation.  The Cambridge O Level has already been phased out, however, and is no longer available in certain administrative regions.

Supporting paragraphs: Georg Friedrich, Prince of Prussia Wilhelm II, German Emperor
Answer: Kaiser