The Concert of Europe was:

a gathering of classical musicians in Vienna.

an alliance to defeat Napoleon.

Napoleon's attempt to control Continental Europe.

Metternich's plan to restore the balance of power in Europe.

The Concert of Europe was Matternich's plan to restore the balance of power in Europe.

Explanation:

Concert of Europe

Concert of Europe was a grouping of European powers including Russia, Prussia, Austria and the United Kingdom. These states had earlier defeated Napolean by forming a Quadruple Alliance in 1815. France later became its member after the restoration of Bourbon monarchy. The system collapsed after the Revolution of 1848. In 1871 German Chancellor Otto von Bismarck restored the system to avoid future conflicts in Europe. This system continued till the beginning of World War I in 1914 when the Concert of Europe split into the Triple Alliance and the Triple Entente.

Who was Matternich?

Matternich was Austrian chancellor from 1821 to 1848 and an influential figure of the time so much so that this era is also known as the Age of Matternich. He wanted to maintain the balance of power in Europe especially by checking Russian territorial ambitions.

Matternich was a central figure in Europe along with Tsar Alexander I of Russia, Talleyrand of France and Lord Castlereagh of the United Kingdom. Concert of Europe was his plan to maintain balance of power in Europe and restraining the ambitions of great powers. This system was also used to oppose revolutionary movements and nationalism in Europe.