I think that it is the second one, Identifying the main idea of a text can help you understand the text better, because I don't believe that a story has to come out and say what the main idea is, lots of times you have to infer from reading. also the third doesn't seem right because most of the time the text should be supported by details or it wouldn't really be a main idea anymore.
so the correct answer should be B.
I hope I'm correct good luck!