Who hosted the Congress of Vienna in 1815 ?
A. Duke Metternich
B. Rainer Ferdinand
C. King Victor Emmanuel
D. Cavour

Hint: The Congress of Vienna was to provide a long-term peace plan for Europe by settling critical issues arising from the French Revolutionary Wars and the Napoleonic Wars.

Complete answer:
The participants were the Great Powers of Austria, Britain, France, Russia, and sometimes Prussia, with limited or no participation by other delegates.

Duke Metternich was at the center of European affairs for three decades as the Austrian Empire's foreign minister from 1809 and Chancellor from 1821 until the liberal Revolutions of 1848 forced his resignation. His quality as a diplomat was commended when his efforts in international diplomacy helped to prevent major wars in Europe. The Congress of Vienna was a meeting of ambassadors of European states chaired by Austrian statesman Klemens von Metternich, and held in Vienna from November 1814 to June 1815.
Archduke Rainer Ferdinand was an Austrian politician who served as Minister-President of Austria from 1861 to 1865. In 1854, Rainer achieved the rank of General major in the Imperial Army and in 1861 was raised to Field marshal lieutenant.
King Victor Emmanuel II was the king of Sardinia till 1861 and later assumed the title of King of Italy. He was the first King of unified Italy since the 6th Century. He fought in the First Italian War of Independence (1848–1849) under his father, King Charles Albert, fighting in the front line at the battles of Pastrengo, Santa Lucia, Goito and Custoza.
Cavour was an Italian statesman and a leading figure in the movement toward Italian unification. After the declaration of a united Kingdom of Italy, Cavour took office as the first Prime Minister of Italy.

The correct answer is option (A).

Note: The Congress of Vienna in real sense wasn’t a congress at all since the members never attended a formal congregation. Rather, all the important decisions were made through informal gathering between few delegates who were mainly the representatives of the Big Four nations.