What type of contract is formed when one party makes a promise in exchange for the act of another?

A unilateral contract is established when one party makes a promise contingent upon the performance of an act by another party. In this type of contract, only one party is bound to fulfill their promise, while the other party does not incur any obligation until they complete the action that triggers the promise. A classic example of a unilateral contract is a reward offer: someone promises to pay a reward to anyone who returns their lost pet. The party offering the reward is obligated to pay once the act (returning the pet) is performed, but the person finding the pet is not obligated to look for it.

In contrast, other contract types involve different dynamics. A bilateral contract requires both parties to exchange mutual promises, where each party commits to fulfilling their end of the agreement. An implied contract is formed through actions or behaviors rather than explicit words, which is distinct from the clear promise scenario in a unilateral contract. An oral contract, while it can be enforceable and involve promises, does not specifically define the nature of the promise in relation to the act of another. Hence, the characteristics of a unilateral contract make it the correct choice for this question.