Which of the following is NOT a characteristic of a valid contract?

A valid contract is generally defined by its essential elements, which include the involvement of competent parties, a lawful purpose, and consideration. The statement regarding the requirement for a contract to be written exclusively is not accurate.

In many circumstances, a valid contract can be oral, as long as it satisfies the fundamental criteria. For example, some agreements, such as those for the sale of goods under a certain value, can be enforceable even if not documented in writing. While having a written contract is often advisable for clarity and to provide evidence of the agreement, particularly in real estate transactions or agreements that exceed a particular duration, it is not a universal requirement for validity.

Thus, the idea that a valid contract must be exclusively in writing does not hold true across all contexts, which makes this statement the one that does not characteristic of a valid contract.