(A) Balkan
(B) France
(C) Germany
(D) ** Serbia
EXPLANATIONS BELOW
Concept note-1: -On 28 July 1914, Austria-Hungary declared war on Serbia. The declaration of war was made after Serbia rejected the terms in an ultimatum sent to them by Austria on July 23, after Archduke Franz Ferdinand, heir to the Austria-Hungary throne, had been assassinated by Serbian student, Gavrilo Princip, in Sarajevo.
Concept note-2: -World War I occurred between July 1914 and November 11, 1918. By the end of the war, over 17 million people would be killed including over 100, 000 American troops. The reason why war erupted is actually much more complicated than a simple list of causes.