Which of the following is NOT a requirement for a valid contract?

Notarization is not a requirement for a valid contract. While certain types of contracts, such as real estate transactions or wills, may need to be notarized to be enforceable, the general principles of contract law do not mandate notarization as a prerequisite for a contract's validity.

For a contract to be valid under common law, it must contain four essential elements: consideration (something of value exchanged), lawful purpose (the contract's subject must not violate public policy or law), capacity (the parties must have the legal ability to enter into a contract, typically meaning they are of legal age and sound mind), and mutual assent (the agreement of both parties to the terms). Notarization is not universally required for all contracts, which is why it does not qualify as an essential requirement for validity.