The correct answer is:
B. Germany's invasion of Poland
Explanation:
In the early stages of WWII, the invasion of Poland in 1939 was the final event that caused the declaration of war of Britain and France against Germany, and for the United States' President Franklin D. Roosevelt, and the American population to think of Germany as the main aggressor. This event didn't immediately caused the entrance of the United States to WWII because of the Neutrality Acts, which prevented the country entering to war because of the previous damage WWI had left, but it began the questioning of whether or not the country should side with the Allies.
President Roosevelt, then asked the Congress to modify the Neutrality Acts, so that the United States could sell guns to the Allies, and later for the Lend-Lease program to be approved to help Britain. The last event that caused United States declaration of war, was Japan's attack on Pearl Harbor on 1941.
