Answer: B. Germany's plan to invade the Soviet Union.
Details:
Germany and the Soviet Union had signed a mutual non-aggression pact in 1939, pledging not to go to war with one another for the next ten years. But in 1941, Hitler broke his non-aggression pact with Stalin and invaded Soviet territory. The German invasion ordered by Hitler was massive way, intending to secure a quick victory. Germany sent over 3 million soldiers, and 3,000 tanks were involved in the attack. The German generals had predicted that they could win in 10 weeks' time. As we know, that didn't happen, and the war on the Eastern front between Germany and the USSR became the bloodiest theater of war in World War II.