Which of the following best describes an offer?

An offer is best described as a definitive expression of willingness to enter into an agreement. This means that when one party makes an offer, they clearly communicate their intent to be bound by the terms of the agreement should the other party choose to accept it. The key aspect of an offer is that it is intended to create a legal obligation upon acceptance, demonstrating the offeror's commitment to the terms laid out.

In legal contexts, this is crucial because it differentiates an offer from other types of communications, such as proposals to negotiate or invitations to treat, which do not carry the same binding intent. A proposal to negotiate suggests that discussions are still open and terms can be altered; a request for information indicates that one party seeks more details without any commitment to agreement; and an invitation to treat, such as advertisements or price lists, invites parties to make offers rather than being a commitment in itself.

Thus, a definitive expression of willingness encapsulates the essence of an offer in contract law, making it the correct answer.