Which topic would be most appropriate for a four- to five-page research paper?

are boys and girls different?
do children learn better in boys-only and girls-only schools?
how many boys-only schools are in the us?
are male teachers or female teachers more effective?

I'd go with the second prompt. The last topic is kind of up to personal preference and there isn't evidence or logic to prove your point without sounding really sexist. The 3rd topic only addresses a fact, so a it would be hard to stretch that topic to 4-5 pages. The first topic is better than the third and fourth, but it's kind of self explanatory. If you choose a topic like how they're psychologically different it could be fine, but you still run the risk of sounding sexist. The second prompt seems more appropriate and is debatable, which makes an essay prompt good. Hope this helps!

I think it would be the second choice, it would have to incorporate all grades and different schools on average.