Which type of beneficiary is NOT entitled to sue the promissor/obligor?

An incidental beneficiary is someone who may benefit from a contract made between two other parties but does not have any legal right to enforce that contract. This means that they cannot sue the promissor or obligor since the contract was not intended to benefit them directly. Their benefit is purely unintentional and incidental, leading to a lack of standing in a legal sense.

In contrast, a donee beneficiary is specifically intended to benefit from the contract and therefore has the right to sue if the promissor fails to fulfill their obligations. Similarly, a creditor beneficiary, who stands to gain from a contract that satisfies a debt owed to them, also has the ability to enforce the contract. Hence, while donee and creditor beneficiaries have rights under the contract, the incidental beneficiary does not, affirming why it is the correct answer in this context.