Answer:
D) Abel's parents' promise is enforceable under a promissory estoppel theory.
Step-by-step explanation:
Promissory estoppel refers to a principle of law which states that an informal promise can be enforceable by law if the promise has led the promisee to rely on it, to his subsequent detriment. This would be the case with Abel, as relying on the promise meant he stayed to work at the farm for 20 years. Therefore, Abel's parents' promise is enforceable under United States law.