Answer:
D. Unenforceable
Explanation:
Preexisting duty is a contractual law whereby there is no consideration given for any changes of the contract and the changes is therefore voidable. Thus, when there's a preexisting duty, it does not constitute valid consideration for a new promise. As such, because consideration cannot consist of performance that the party had a pre-existing duty to perform, it makes new contracts unenforceable.
