A, B, C, D
(everything except the last answer)
Assuming you're referring to World War I, the main event that moved the United States from neutrality to war was the sinking of the Lusitania, since many Americans died, and it was the Germans who were responsible.
The Truman Doctrine basically said that America would provide help even military help to any country that was under threat of being taken over by communism. By contrast, the Marshall Plan provided aid in the form of food and money to countries in Western Europe whether they were being threatened by communism or not.