Which type of contract binds only one party to act?

A unilateral contract is a type of agreement where only one party is obligated to perform their part of the deal, while the other party is not bound to any contractually enforceable action. This type of contract typically involves a promise from one party in exchange for a specific act by another party. An example of a unilateral contract is a reward contract, where one party promises to pay a reward for the return of a lost item; only the person returning the item is required to act to receive the reward, while the person offering the reward is not obligated to pay until the condition (returning the item) is fulfilled.

In contrast, a bilateral contract requires both parties to fulfill their obligations, such as a traditional sales agreement where the buyer promises to pay for property, and the seller agrees to transfer the property. A conditional contract involves the performance being contingent on specific conditions being met. A sales contract generally refers to a bilateral agreement where ownership is transferred, but it does not specifically highlight the one-sided obligation an unilateral contract embodies.

Thus, the defining nature of a unilateral contract, where only one party is bound to act, makes it the correct choice in this context.