Which of the following is required for a contract to be legally valid?

For a contract to be legally valid, it is essential that it involves a legal product or service. This requirement ensures that the content of the contract does not promote or involve illegal activities, which would otherwise render the contract void. Contracts that enforce or stipulate illegal acts violate public policy and cannot be upheld in a court of law. Hence, this condition serves as a fundamental principle in contract law, ensuring the legality and enforceability of the agreement.

The other options, while related to elements of verbal agreements, age of parties involved, and formalities such as witnessing, do not universally apply to the basic legality of a contract. For instance, contracts can be valid even if they are not verbally agreed upon, as written contracts are common and enforceable as well. Additionally, involving a minor may complicate aspects of contract enforceability, but it does not affect the legality of the product or service being contracted. Lastly, the presence of a formal witness is not a requirement for all contracts. Therefore, the necessity for the contract to revolve around legal products or services stands as the critical criterion for legal validity.