Answer: A) It reinforces the idea of the United Nations’ strength and power.
Repetition is often used as a rhetorical device in order to emphasize an idea. It can also be used to provide more weight to a particular topic or to leave a long-lasting image in the listener's mind. In this case, the idea that Roosevelt is trying to emphasize is that the United Nations will strike again, because of its strength and power. By repeating the word "strike," he is making sure that the listener understands the extent of this promise and the power of the UN.