Marge promises her 21-year-old son, Bart, that she will pay him $100 if he quits smoking for a month. Help me?

If this is a legal question, we need to know which country's law applies. In English law, for instance, for there to be a binding contract there have to be at least these three elements: offer, acceptance, and consideration (money). The closest I can come with these on your question would be (A) because Marge offered, Bart accepted but he "gave no benefit to Marge" might be (?US) equivalent of English "consideration" which is missing here, so only 2 out of 3, so no legally binding contract. Wherever you are, you need to check the law on contracts, and then see which of the four options apply. I think B C and D are red herrings