Since the U.S. was fighting against communist nations in WW ll, I'd say answer A. That was when the U.S. first realized how dangerous communism is, since the whole nation in a sense follows the wants of one man.
Fighting against Hitler, Mussolini, and the other guy (I can't remember who it was) really opened America's eyes to the horrors of communism, which is why they decided to strengthen the Democratic side of America.
I would say the cold war. All throughout the cold war, we saw the effects that communism had on Russia, and I think that seeing that pushed us more towards a democracy.
Hope this helps. If I'm wrong, please comment and rate low. No that's not a joke.
