Which of the following is a key requirement for a valid contract?

A key requirement for a valid contract is mutual assent, which refers to the agreement and willingness of the parties involved to enter into the contract. This concept is often expressed through the terms "offer" and "acceptance." For a contract to be enforceable, both parties must understand and agree to the terms presented. This mutual agreement indicates that both parties have a meeting of the minds, allowing the contract to be valid and binding.

The other options, while they may play a role in certain types of agreements or specific legal contexts, are not universally necessary for a contract's validity. For example, the presence of a witness, filing with the court, and notarization may be requirements in specific situations or for particular types of contracts, but they do not constitute a fundamental requirement for the validity of all contracts. Therefore, mutual assent stands out as the essential element that must be present for a contract to be recognized legally.