Final answer:
Carl is an (Option a)undisclosed principal because he remains completely unknown to the seller, while Diane acts as if she is the buyer for herself.
Step-by-step explanation:
In the described scenario, Carl hires Diane to act as his agent in the purchase of an office building, and Carl wishes to remain unknown to the seller. Since Diane represents herself as the buyer, and the existence of Carl as the principal is completely unknown to the seller, Carl is an undisclosed principal. An undisclosed principal is a party in an agency relationship where the existence of the principal is not known to the third party that the agent interacts with. In contrast, in the case of a partially disclosed or fully disclosed principal, the third party is aware of the fact that the agent is acting on behalf of someone else, even though the identity might not be fully revealed in a partially disclosed situation.