What type of contract involves a promise given by only one party?

A unilateral contract is characterized by a promise made by one party in exchange for a specific action or performance from another party. This means that only one party is obligated to fulfill the promise, while the other party's acceptance is expressed through their actions. A common example of a unilateral contract is a reward offer, where one party promises to pay a reward for a lost item, and the obligation is fulfilled only when someone finds and returns the item.

In contrast, a bilateral contract involves mutual promises between two parties, meaning both parties are bound to perform their respective promises. Enforceable and unenforceable contracts refer to the legal validity and ability to be upheld in a court of law, rather than the nature of promises involved. Thus, the defining feature of a unilateral contract is the one-sided promise, making it the correct choice in this context.