Hi, the flaw here is that the author wrongly assumes that any action that is motivated at least in part by desire for praise is motivated entirely by desire for praise. So, we need an answer choice that proves that if praise is one of the factors, then it is the only factor. B is a conditional that tells us that if an action is only motivated by desire for praise then it is not praiseworthy. It's sufficient condition is the thing we are trying to prove, so it is not correct.
