The correct answer is A. The US started supplying countries that were fighting Nazi Germany.
From 1939 to 1941, the American role in World War II evolved significantly. Initially, the United States maintained a policy of neutrality, but as the war progressed and the threat posed by Nazi Germany became more apparent, the U.S. began to support Allied nations through measures like the Lend-Lease Act, which allowed for the supply of military aid and resources to countries fighting against the Axis powers. Public opinion indeed shifted during this time, but the primary change in American involvement was the increasingly active support of nations like Britain and China against the Axis forces.