The best answer is A) Because war was forced upon them by Hitler
Stalin was no idealist, and had no issue with letting Hitler run amok in Europe, the Mediterranean, and the Middle East. He pursued a neutrality agreement with Japan and, in 1940, was in the process of negotiating a similar treaty with Hitler. Stalin wanted some non-German European territory out of the deal, and Hitler decided that he'd prefer to invade the Soviet Union.
In this way, Germany forced the USSR to ally with the Allies, although it could very well have sided with the Axis if things had turned out differently.
Best answer is A
