Answer:
The best answer is indeed letter D. Many American men were drafted during World War II, and many other men volunteered to serve.
Explanation:
When combining independent sentences, we must take a few things into consideration. Proper punctuation and conjunction must be chosen. Also, parallelism - the repetition of a certain grammatical form - is important for the sentence to sound more fluid.
The first two options present problems with parallelism since the two main verbs are used in different ways - were drafted X volunteering. The third option corrects that problem since volunteering becomes volunteered - now there is parallelism since both clauses are in the simple past. But the conjunction "or" conveys an idea that one of two possibilities can be realized. The best conjunction in this case would be "and", since it conveys a sense of addition - both things happened, men were drafted, and men volunteered. Therefore, the best option is letter D. Many American men were drafted during World War II, and many other men volunteered to serve.
