Final answer:
An acceleration of the performance of the duty is not an exception to the preexisting legal duty rule.
Step-by-step explanation:
The preexisting legal duty rule states that a promise to perform a preexisting legal duty is not valid consideration for a contract. This means that if a person is already legally obligated to do something, their promise to do that thing is not enforceable as a separate contract. In the given options, d) An acceleration of the performance of the duty would not be an exception to the preexisting legal duty rule. This is because accelerating the performance of a duty does not create a new promise or consideration separate from the existing duty.