Which of the following are requirements for a valid contract?

The correct answer identifies the essential elements that constitute a valid contract. A valid contract typically requires an agreement, which reflects mutual consent between parties; consideration, which is the value exchanged between parties; capacity, meaning that all parties must have the legal ability to enter into a contract; and legality, which ensures that the contract's subject matter is lawful.

These elements are foundational in contract law. Without an agreement, there's no basis for a contract. Consideration is equally crucial as it differentiates a contract from a gift; both parties must provide something of value. Capacity confirms that parties are of a legal age and sound mind, thus able to understand the implications of their agreement. Lastly, legality ensures that the contract does not violate any laws, which inherently affects its enforceability.

The other options do not encompass all necessary elements for a valid contract. Some mention aspects that might pertain to specific types of contracts or additional requirements that are not universally required. Thus, while they may contain relevant terms, they do not fully represent the core characteristics that define the validity of a contract.