Which of the following is an essential element of a valid contract?

A valid contract is fundamentally grounded in mutual agreement between the parties involved. This element is critical because it signifies that both parties have come to a consensus about the terms and conditions laid out in the contract. Mutual agreement, often demonstrated through an offer and acceptance, is what establishes the contractual relationship, ensuring that each party understands and agrees to what is being promised or required.

The presence of goodwill can enhance the relationship between the parties and may foster a more cooperative environment, but it doesn't constitute a legal requirement for a contract's validity. Similarly, while some contracts may require formal documentation for enforceability, not all contracts must be in writing; oral contracts can also be valid under certain circumstances. Lastly, while a public notary signature can be beneficial in certain legal contexts, it is not an essential component for most contracts to be considered valid. Therefore, mutual agreement stands out as a fundamental element in the formation of a binding contract.