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

To determine why the correct answer is that a formal writing signed by both parties is not a requirement for a valid contract, it's important to understand the foundational elements that constitute a valid contract.

A mutual agreement between parties is essential, typically established through an offer and acceptance, demonstrating that both parties have a clear understanding and consensus on the terms of the contract. Consideration, which refers to something of value exchanged between the parties, is also a crucial requirement. This ensures that each party is providing something in return for the benefits they receive.

Legality of the contract's purpose is another critical element. A contract must have a lawful objective; agreements for illegal acts cannot be enforced.

However, the requirement for a formal writing signed by both parties does not apply universally to all contracts. While certain types of contracts, such as those involving real estate or that cannot be performed within one year, must be in writing to be enforceable under the Statute of Frauds, many contracts can be valid and enforceable even if they are oral agreements. Therefore, it is not a necessity for all contracts to be in writing, which makes this the correct answer.