Final answer:
Frank's acceptance of Eve's offer in a unilateral contract is complete when he unloads the truck, which is the act required by the offer.
Step-by-step explanation:
The question deals with the concept of contract formation and when an acceptance of an offer is considered complete in the context of a unilateral contract. In the scenario described, Eve has made a unilateral offer to Frank, which is a promise of payment in exchange for the performance of an act. Frank's acceptance of this kind of offer is complete when he fully performs the act required by the offeror. Therefore, Frank's acceptance is complete (a) When he unloads the truck, as this constitutes the completion of the performance required by Eve's offer.