What is the main characteristic of a unilateral contract?

A unilateral contract is characterized primarily by its reliance on one party's promise in exchange for the performance of an act by another party. This means that one party makes a promise that the other party can accept by performing a specific action, without the need for that second party to make a promise in return.

For example, if someone offers a reward for the return of a lost item, that person is making a unilateral promise. The contract is formed when someone finds the item and returns it, thereby fulfilling the condition of the promise. This highlights that the crux of the agreement revolves around the act performed rather than a mutual exchange of promises, which is what distinguishes it from bilateral contracts.

Other options do not align with the nature of unilateral contracts. While there may be circumstances where witnessing a contract is necessary, it's not a defining characteristic of unilateral contracts specifically. Similarly, a requirement for consideration from both parties is more aligned with bilateral contracts, and the premise of promises being made by two parties does not appropriately capture the essence of a unilateral agreement. Thus, the fundamental characteristic is the existence of a single promise that is contingent upon action, making choice B the correct answer.