In which type of contract can acceptance only occur through performance?

In a unilateral contract, acceptance is specifically tied to the performance of a specific act. This type of contract involves one party making a promise in exchange for an act by another party. The contract becomes binding only when the action is performed. For example, if someone offers a reward for the return of a lost pet, the contract is formed only when someone finds and returns the pet, thereby accepting the offer through their performance of that act.

In contrast, a bilateral contract involves mutual promises, meaning both parties agree to perform certain obligations. An executed contract refers to a contract that has been fully performed by both parties, while a formation contract is not a standard legal term used in contract law. Thus, only in a unilateral contract is acceptance conditional upon the performance of a specified act, making it the correct answer.