Question: Which German Prince, born in 1818, composed Diana von Sloange?
Context: Diana von Solange is an opera by the German prince Ernst II of Saxe-Coburg-Gotha, an amateur composer.  It was premiered in 1858.  In 1859, Franz Liszt wrote an orchestral "Festmarsch nach Motiven von E. H. z. S-C-G", S.116, based on themes from "Diana von Solange" (E. H. z. S-C-G was short for Ernst Herzog zu Sachsen-Coburg-Gotha). Prince William of Saxe-Gotha-Altenburg (German: "Wilhelm Carl Christian von Sachsen-Gotha-Altenburg"; 12 March 1701, Gotha - 31 May 1771, Tonna) was a German prince of the Saxe-Gotha-Altenburg house, a junior line of the Ernestine Wettins.  He served as a Generalfeldzeugmeister in the armies of the Holy Roman Empire. Heinrich XLII (1752–1818) was a German prince of the House of Reuss. Prince Christian of Hesse (Danish: "Christian af Hessen" ; German: "Christian von Hessen" ) (14 August 1776 – 14 November 1814) was a German prince and member of the House of Hesse-Kassel.  As a son of the Danish Field Marshal Prince Charles of Hesse-Kassel and Princess Louise of Denmark, he was a member of the extended Danish Royal Family and spent his entire life in Denmark. Prince Wilhelm of Urach, Count of Württemberg, 2nd Duke of Urach ("Wilhelm Karl Florestan Gero Crescentius"; German "Fürst Wilhelm von Urach, Graf von Württemberg, 2.  Herzog von Urach"; 30 May 1864 – 24 March 1928), was a German prince who was elected in June 1918 as King of Lithuania, with the regnal name of Mindaugas II.  He never assumed the crown, however, as German authorities declared the election invalid; the invitation was withdrawn in November 1918.  From 17 July 1869 until his death, he was the head of the morganatic Urach branch of the House of Württemberg. Ernest II (German: "Ernst August Karl Johann Leopold Alexander Eduard"; 21 June 1818 – 22 August 1893) was the sovereign duke of the Duchy of Saxe-Coburg and Gotha, reigning from 1844 to his death.  Ernest was born in Coburg as the eldest child of Ernest I, Duke of Saxe-Coburg-Saalfeld, and his duchess, Princess Louise of Saxe-Gotha-Altenburg.  Fourteen months later, his younger brother Prince Albert was born, who became consort of Queen Victoria of the United Kingdom.  Ernest's father became Duke of Saxe-Coburg and Gotha in 1826 through an exchange of territories. Alexander of Courland (German: "Alexander von Kurland" ) (18 October 1658 – 1686), nicknamed "Alexander the One-armed", was a Baltic German prince and the brother of Duke Frederick (II) Casimir of Courland. He was the youngest son of Jakob Kettler (1610–1682), Duke of Courland and his wife Luise Charlotte von Brandenburg (1617–1676), oldest daughter of Prince elector Georg Wilhelm von Brandenburg.  He was a member of the Kettler dynasty. The title Countess von Gleichen was created by the Duke of Saxe-Coburg and Gotha in 1861 for Laura Seymour, morganatic wife of Prince Victor of Hohenlohe-Langenburg, a German prince and nephew of the British Queen Victoria.  Prince Victor is sometimes also identified as Count Gleichen, "jure uxoris".  After Princess Victor's death her title passed to her only son Count Edward.  In 1917, when British citizens were required to relinquish all German titles, the Gleichen family retained their name but gave up their comital status, instead taking the precedence of children of a British marquess, allowing them to be styled Lord and Lady.  This title and status had earlier been given to Princess Victor when her brother had inherited the title Marquess of Hertford. August Victor Louis of Saxe-Coburg and Gotha (German: "August Viktor Ludwig" ; 13 June 1818 – 26 July 1881), was a German prince of the Catholic House of Saxe-Coburg and Gotha-Koháry of the house of Saxe-Coburg-Gotha.  He was a General Major in the army of Saxony and the owner of Čábráď and Štiavnica, both in modern-day Slovakia. Frederick Ferdinand of Anhalt-Köthen (25 June 1769, Pless – 23 August 1830, Köthen) was a German prince, Ascanian ruler of the principality of Anhalt-Pless and, from 1818, of the duchy of Anhalt-Köthen.  He was the second son of Frederick Erdmann, Prince of Anhalt-Pless, and his wife, Louise Ferdinande, daughter of Henry Ernest, Count of Stolberg-Wernigerode.
Answer: Ernst II of Saxe-Coburg-Gotha