A valid agency requires a(n):

While many real estate agency relationships in California do require written agreements (such as buyer representation agreements and listing agreements under the Statute of Frauds), a written agreement is not universally required for a valid agency to exist. General agency law recognizes oral and implied agencies as legally valid, making 'written agreement' an overly restrictive and incorrect answer.

An attorney-in-fact is a specific type of agent who holds a power of attorney — a formal written document granting legal authority to act on someone's behalf in legal and financial matters. While an attorney-in-fact is one example of an agent, they are not required to create a valid agency relationship; most agency relationships exist without any power of attorney.

The answer is not 'None of the above' because option B (express agreement) is a correct statement of law. 'None of the above' is a trap for test-takers who overthink the question or confuse the requirements for specific types of agency agreements with the general requirements for any valid agency.