Answer:
B. A system of alliances obliged nations to support one another.
Step-by-step explanation:
By 1910, the majority of European states were part of one of the great opposing alliances: Central Powers being Germany and Austria-Hungary, and Allies, that were France, Russia and Great Britain.
As this system consisted of a compromise that obligated nations to respond in case of agressions. As Serbia and Russia were very deep related and had a tradition of friendship as allies, they declared war on Austria Hungary when the troops of this came to opress Serbia.
Germany in turn declared war on Russia, its natural rival on East Europe.
Finally France , to stay with Russia, declared war on Germany