After Austria-Hungary declared war on Serbia, Russia:

A. ordered the assassination of the Austro-Hungarian leader.

B. formed a pact with Germany to support Austria-Hungary.

C. convinced France to launch a surprise attack against Germany.

D. mobilized its military because of its alliance with Serbia.

Since Russia had an alliance with Serbia, it ordered its forces to mobilize for war. it wasn't a formal alliance, but it did specify that if someone declared war on Serbia, Russia would also step in. This resulted in other countries also getting involved in the war. The answer would be D!

-TetraFish