The correct option is B Austria-Hungary
Austria-Hungary made first declaration of war against Serbia because of assassination of the
Austrian Prince Francis Ferdinand and his wife Sophia on June 28, 1914. The Archduke Ferdinand along with his wife
Sophia were assassinated by a student of the ‘Black hand’, a revolutionary-organization,
at Sarajevo, the Capital of Serbia. Austria served an ultimatum on Serbia on July 23 making eleven demands. Serbia accepted most of them except those which would have led to the loss of her sovereignty. Austria declared war on Serbia on July 28, 1914.