C: rewarding a child after an activity he enjoys may create too many
    reasons to enjoy the activity.

    Correct.

The overjustification effect states that when we reward a person for doing something that they already enjoy (their enjoyment is intrinsic) they will like it less.  This answer is correct, because the overjustification (e.g., over= [too much] justification) states that in this situation a child who originally had an intrinsic motivation for enjoying an activity, now also has an extrinsic motivation for liking the activity (i.e., the reward).