C. Communism
While (i believe) there were communists during the time, the main cause of WWI was the other 3.
Alliances: because of the assassination of Archduke Ferdinand, Austria-Hungary declared war on Serbia, which sparked the chain of events that led to a global conflict.
Militarism: The build up of arms and armies as well as the conquering and acquiring of new territories led to both sides fighting for the same lands.
Nationalism: Without Nationalism, there would be no support for the war effort, leading to a deescalation of the war.
hope this helps