Final answer:
If the promisor fails to perform a contract, a donee third-party beneficiary whose rights have vested can sue the promisor. The promisee is the one with the original agreement, but the beneficiary's right to sue arises from the intended benefit they were to receive.
Step-by-step explanation:
Under contract law, when a promisor fails to perform a contract obligation, a donee third-party beneficiary with vested rights generally has the right to sue the promisor. The donee beneficiary was not part of the original contract but stands to gain a benefit from the contract's performance and thus acquires rights to enforce the contract once their rights have vested.
The promisee is the person or entity that initially made the agreement with the promisor to benefit the third party. The promisee can sue for breach of contract as well, but typically, the donee beneficiary is allowed to take legal action directly against the promisor.
The correct answer to the question, therefore, is B: The promisor. They are the ones who have undertaken the obligation and directly failed the third-party beneficiary by not performing under the contract.