We need a question that isn't too narrow, and isn't too broad.
For example:
"What started WWII?" is too broad (needs a book to be answered properly), and "What is a general in military?" is too narrow (can be answered in a sentence).
With this information, answer A is incorrect, too narrow.
Answer B is a good question that can lead to more smaller questions and lead to some good paragraphs.
Answer C is incorrect, being too narrow.
And finally answer D is incorrect being too broad a subject to talk about, too complex if you want to explain it properly.
Answer B is the correct answer choice for this question.