Final answer:
The client in a single agency relationship in a real estate transaction is the buyer, who is also referred to as the principal.
Step-by-step explanation:
In a single agency relationship with regard to a real estate transaction, the principal is the client. Therefore, the correct answer is C) The buyer (The principal). In this type of relationship, a real estate agent or broker represents only one party in the sale of real estate. If the broker is working with the seller, they are the seller's agent, and if they are working with the buyer, they are the buyer's agent. They owe fiduciary duties to their client, including loyalty, confidentiality, and the duty to act in the best interests of their client.