Answer: B. Sam will lose because his only right of action is against Trudy.
Step-by-step explanation:
Sam had no agreement with Betty to pay him for the book. His only agreement was with Trudy and so he has no right of action against Betty. Had Trudy made him a party to the transaction with Betty, then he would have had a right of action against Betty.
His only option is to go after Trudy as she will not be able to avoid paying him because she delegated the debt, especially as she didn't tell Sam about it.