Answer:
promissory estoppel
Step-by-step explanation:
Based on the information provided within the question it can be said that in this scenario Kenneth can sue his parents under the doctrine of a promissory estoppel. This is a doctrine which prevents an individual/group from abiding to a promise, even if that promise was not made into a legal written contract. Such as is the case in this scenario since Kenneth's parents promised to deed him the ranch.