What are advantages and disadvantages of human trafficking?

C. At what age is it most appropriate to teach a foreign language

Explanation:

The question is asking Which would be the best question to use on a Research essay.

A. is incorrect because even though yes you'd have to do research on it most research essay's are either a more of a debate based research or  have multiple opinions from other professionals and can have many different views on the topic, yet this topic is asking us of one thing can can only have one opinion/answer.

Using that information we can eliminate, B. and D. because B. is asking what happens in death of a salesman, which the reader can just go and read that book/article/information, that'd be more of a summary not a research essay. D. can be eliminated because its asking about which nations were the main combatants in WW1, though this one seems most logical when you think of the research you'd need to do remember what we said about A. a research paper needs to include something that can be almost debated something that has a solid answer but can have more than one solid answer at the same time, that is why I believe D. is incorrect.

I hope this helped, have a good day. :)