For Donna's common law agency relationship with Chip to be legal, what must be true?

The correct choice indicates that for Donna's common law agency relationship with Chip to be legal, her brokerage must exclusively practice single agency. In a single agency relationship, a broker represents only one party in a transaction, ensuring loyalty and advocacy for that client without conflicting interests from representing another party.

When a brokerage practices single agency, it establishes a clear boundary in which the agent's loyalty is solely with one client, preventing potential conflicts of interest. This is essential for the integrity of the agency relationship and is a fundamental principle in real estate practices, particularly under common law.

In contrast, the other options imply situations that could present potential conflicts or are not a requisite for establishing a legal agency relationship. For instance, merely disclosing the agency relationship to Chip does not fulfill the legal requirements if the brokerage practices dual agency, nor is written permission from the broker necessary if the agency is already aligned with single agency practices. Hence, emphasizing that a brokerage exclusively practices single agency reinforces the validity and legality of the agency relationship under common law.