Which of the following is NOT a key element of a valid contract?

A valid contract typically requires several key elements to ensure that it is enforceable and legally binding. These elements include an offer and acceptance, which establishes the mutual agreement between the parties involved; competent parties, ensuring that all individuals entering the contract are legally able to do so (i.e., of sound mind and legal age); and a legal purpose, meaning that the contract must be for a lawful activity.

The aspect of a written document, while often recommended for clarity and as evidence of the agreement, is not always a legal requirement for a contract to be valid. Many contracts can be enforceable even if they are oral, provided they meet the key elements mentioned above. However, some types of contracts—like those concerning real estate or that cannot be performed within a year—are required by law to be in writing under the Statute of Frauds. Consequently, the requirement for a written document is situational and not a universal element of a valid contract. This is why it is the correct answer as a choice that is NOT a key element of a contract.