(A) Belgium
(B) ** Bulgaria
(C) Serbia
(D) Russia
EXPLANATIONS BELOW
Concept note-1: -The Allies described the wartime military alliance of Germany, Austria-Hungary, Bulgaria and the Ottoman Empire as the ‘Central Powers’. The name referred to the geographical location of the two original members of the alliance, Germany and Austria-Hungary, in central Europe.
Concept note-2: -The Central Powers included Germany, Austria-Hungary, the Ottoman Empire and Bulgaria.
Concept note-3: -Bulgaria was important for both belligerents because of its strategic geo-political position on the Balkans and its strong army. If Bulgaria entered the war on the side of the Central Powers then Serbia would have been defeated, which could influence the still neutral Romania and Greece.
Concept note-4: -The Central Powers, also known as the Central Empires, was one of the two main coalitions that fought in World War I (1914–1918). It consisted of the German Empire, Austria-Hungary, the Ottoman Empire, and the Kingdom of Bulgaria and was also known as the Quadruple Alliance.
Concept note-5: -During the summer of 1915, when the military balance swung in Germany’s favour, Bulgaria committed to the Central Powers and declared war on Serbia on October 1 (October 14).