Which of the following is crucial for a contract to be enforceable?

A contract's enforceability is primarily hinged on the presence of mutual consideration, which refers to the value that is exchanged between parties involved in the contract. This exchange can be in the form of money, services, goods, or a promise to do something. The concept of consideration ensures that both parties have a stake in the agreement and that they are in a reciprocal relationship, which is a foundational aspect of contract law.

While a written and signed document can add clarity and strengthen evidence of the agreement (as suggested in the first option), it is not strictly necessary for all contracts to be enforceable, particularly in oral agreements where consideration is present. Knowing each other, as mentioned in the third option, is not a legal requirement for a contract to be valid or enforceable. Similarly, involving a third party does not inherently affect the enforceability of the contract between the original parties unless specific obligations concerning the third party are part of the agreement. Therefore, mutual consideration is vital for establishing a legally binding contract.