Which type of contract requires one party to fulfill a promise in exchange for an act from another party?

A unilateral contract is characterized by one party making a promise in exchange for the act or performance of another party. This type of contract becomes binding when the other party completes the requested act. An example of this would be a reward contract, where one party promises to pay a sum of money to anyone who finds and returns a lost pet. Here, the promise to pay is conditional upon the act of returning the pet.

In contrast, a bilateral contract involves mutual promises between two parties, where each party is both a promisor and a promisee. A conditional contract contains stipulations that must be met for the contract to be enforceable, while an aleatory contract is based on uncertain events and typically involves risk, where the performance depends on the occurrence of a specified event. Understanding the definitions and characteristics of these contract types highlights why a unilateral contract is the correct answer.