The correct option is D Bulgaria
The Central Powers, consisting of Germany, Austria-Hungary, the Ottoman Empire and Bulgaria also known as the Quadruple Alliance was one of the two main factions during World War I (1914-18). It was defeated by the Allied Powers that had formed around the Triple Entente.
The Triple Entente refers to the understanding linking the Russian Empire, the French Third Republic, and the United Kingdom of Great Britain and Ireland. The understanding between the three powers was a powerful counterweight to the Triple Alliance of Germany, Austria-Hungary, and Italy.