While C is clearly a true statement, it is not the best answer for this question.  Instead, the best answer to this question is D.

Germany, Japan, and Italy were the main Axis powers for most of the war.  Japan was not officially at war until December of 1941, but it did have an alliance of sorts with Germany and Italy.  Therefore, C is right.  However, both A and B are right as well.  All three of these powers wanted to expand their territory.  Because of this, they forced the Allies to fight in practically every part of the world.  There was fighting in Africa and in Europe and in mainland Asia as well as the islands of the South Pacific.  Therefore, D is the best answer.