What is the moral of a fable?

A) It is an idea that is common to all people

B) It is a generalization about the plot

C) It is the message of the fable

D) It is a thought about the characters

C is the answer you are looking for. A common idea isn't a moral, the generalization is the Main Idea, but not quite the moral. The thought of the characters almost never have the moral in them. Therefore, C is your answer.