contract right act of 3rd party

R’s wife K expressed the wish that R, if he survived her, should have the use of her house. After K’s death her executor agreed to allow R to occupy the house a) because of K’s wishes and b) on the payment by R of $24 per year

R seeks to enforce this agreement and the Executor wishes to avoid it in order to sell the house. What is the legal position?

a) R can enforce the agreement on the basis of his deceased wife’s wishes
b) R cannot enforce the agreement because the promise to pay is not consideration
c) R can enforce the agreement because the promise of $24 per year provides consideration for it
d) R cannot enforce the agreement because the promise of $24 per year is not sufficient consideration.

Sir, correct answer is C, but please explain me that why the answer cannot be a) also? Because according to contract rights of 3rd party act 1999, successor R can also enforce the contract as the contract was for his benefit and he was expressly identified in it?