Kenneth's parents own a cattle ranch. when they become too elderly to run the ranch by themselves, kenneth moves back to the ranch to help them. with his parents' help and permission, kenneth builds a house on the ranch. after he builds it, his parents refuse to deed him the land. kenneth can sue his parents under the doctrine of:

a. unfair accord.

b. satisfaction.

c. promissory estoppel.

d. covenant not to sue.