Which of the following is true about a valid contract?

A valid contract is characterized by the presence of an offer and acceptance. This essential component establishes a mutual agreement between the parties involved, which is foundational to contract law. An offer is a proposal to enter into an agreement, while acceptance is the acknowledgment and agreement to the terms of that offer.

The other aspects listed, while they can be important in certain contexts, are not universally requisite for all contracts. For example, some contracts do not necessarily need to be in written form or require witnesses to be considered valid, depending on the type of agreement and the jurisdiction. Similarly, while it is generally good practice to have all parties sign a contract, there are scenarios where an agreement may be valid even without every party's signature, particularly in informal arrangements or implied contracts.

Thus, the presence of an offer and acceptance is a core characteristic that ensures a contract is not only valid but also enforceable.