Which of the following is an example of a unilateral contract?

A unilateral contract is characterized by a one-sided promise, where one party makes a promise in exchange for an act or performance by another party, but that other party is not obligated to act. In the given options, a promise to pay for a service after it is completed exemplifies a unilateral contract because one party (the one making the promise to pay) is making a commitment contingent upon the other party fulfilling their end by completing the service provided.

In such a scenario, the party offering the payment is not bound until the service is performed, highlighting the essence of a unilateral contract where only one party is bound to perform as long as the other fulfills the specified condition. This aligns with the definition of a unilateral contract, where the offeror guarantees a payment or reward upon the fulfillment of a specific action by the offeree.

The other options involve either mutual obligations or contract types that do not fit the criteria of unilateral contracts, as they imply a reciprocal agreement where both parties have duties to perform. For instance, a listing agreement typically constitutes a bilateral contract, wherein both the agent and the principal have obligations.