Which of the following best indicates that Harold is Kelsey's seller client?

The best indication that Harold is Kelsey's seller client is the existence of a signed agency agreement between them. This formal document establishes a legal relationship wherein Harold, as the seller, authorizes Kelsey to act on his behalf in the sale of his property. An agency agreement outlines the duties and obligations of both parties, providing a clear framework for representation.

In contrast, an oral agreement, while potentially valid, lacks the formalized nature and protections that a written contract offers. Having a signed listing with another agent suggests that Kelsey is committed to a different seller, thus negating Harold's client status. Lastly, being related does not inherently signify a professional relationship pertaining to real estate representation unless supported by a formal agreement. Therefore, the signed agency agreement is the strongest and most definitive evidence of Harold being Kelsey’s seller client.