C. He is pointing out the error in the audience's logic, which will motivate it to act.
In this particular speech, Roosevelt is speaking out against the general population’s view that the United States of America should stay out of a war and, for all intents and purposes, be self-quarantined. What he does here is mention the consequences of being isolated and potentially letting evil dominate the world. Thus, he is pointing out the folly of his audience’s belief and logic.