Which characteristic defines a unilateral contract?

A unilateral contract is specifically defined by having only one party make an enforceable promise. In this type of contract, one party, the offeror, makes a promise that the other party, the offeree, can accept by performing a certain action. For instance, a common example is a reward contract where one party promises to pay a reward to anyone who performs a specific action, such as finding a lost pet. The enforceable promise lies solely with the person offering the reward, while the person accepting the offer does so through their actions, not by making a promise in return.

This characteristic differentiates unilateral contracts from bilateral contracts, where both parties exchange enforceable promises. The nature of unilateral contracts means that the promisor is bound only when the offeree completes the required action, but the offeree does not have any obligations until they choose to accept the offer through their performance. This is what makes option B the defining characteristic of a unilateral contract.