Which example best illustrates a unilateral contract?

A unilateral contract is defined by an agreement where one party makes a promise or offers something of value, and the other party can accept that offer simply by performing a specific action, rather than by making a counter-promise. In the context of a reward for finding a lost pet, the offering party is making a promise to pay a certain amount if someone successfully finds and returns the pet. Here, only the person offering the reward is making a promise, while the individual who finds the pet is not obligated to return the pet or claim the reward—thus illustrating a unilateral nature.

The other examples provided do not fit the definition of a unilateral contract. In the case of the purchase agreement, both parties are making promises to one another, which constitutes a bilateral contract. A landlord and tenant signing a lease also represent a bilateral contract, as both parties agree to specific terms and obligations. Finally, a couple discussing wedding plans involves mutual discussions and plans but does not involve a unilateral promise or commitment from one party to the other. Therefore, the offering of a reward effectively demonstrates the core characteristic of a unilateral contract.