Which topic would be most appropriate for a four - to five-page research paper? A. Are boys and girls different? B. Do children learn better in boys-only and girls-only schools? C. How many boys-only schools are in the US? D. Are male teachers or female teachers more effective?

The answer is letter B. Do children learn better in boys-only and girls-only schools?

It is sufficiently specific enough to pose as a research question but also broad enough to add something novel to the literature. Also, the other choices carry purely descriptive research designs (How many?), while B attempts to measure correlation (Is learning better here or here?). This would warrant a longer research of 4 to 5 pages.

I think B would be the best because you can go into more detail and get more data and information on that subject.