I believe your answer is C.
For A, we were never in the League of Nations in the first place, so why would we want to enforce any policies related to it?
For B, this is was in no way to stimulate the economic growth of the US. That's is the obviously wrong answer.
For D, The Neutrality Acts wanted nothing to do with the war. It didn't care if they be any peacekeeping troops in Europe. It supported to keep the US out of the war, and that's all.