The correct answer is A) because people of the world -including dissenters in Germany- had approved their cause.
In Stalin's opinion, the USSR decide to go to war with Germany because people of the world -including dissenters in Germany- had approved their cause.
Joseph Stalin was a dictator. The people that surround him, his advisors, often said yes to everything he said. So in Stalin's mind, the Soviet Union had to wage war against Germany because he thought he had total support from the people in Russia; even dissenters in Germany. So he considered that he was fighting the tyrant Hitler and his expansionist ideas that included an invasion to the USSR territory as he did.
The other options of the question were b) because Britain would be fighting with the Soviets, c) because Soviet troops had received threats from German ships, and d) because Hitler took Polish territory when he broke the nonaggression pact.
