B: rewards are harmful only when they are given as a surprise

    Incorrect.
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.  Rewards for doing something, that are promised to people who already like doing it, can diminish intrinsic motivation, making your play seem like work.  If rewards are given as a surprise at the end of a task, and are unanticipated by those who will receive them, a reward does not reduce intrinsic motivation.  Since this contridicts choice B, it is incorrect.