Hello, Chandreka!
The correct answer is (b) - oversimplification. While it is true that listening carefully in class is part of what you need to do to earn good grades, the situation is more complicated than that. You also need to study the material at home, for example. You may also need to ask questions in class if there are some things you don't understand. So, the statement is making something simple that is not really all that simple (how to get good grades), and so that tells you that it is an example of oversimplification.
I hope that's a helpful answer!
Jeff M.