If a broker allows a person to act as her agent, which concept prevents her from denying the agency?

The concept that prevents a broker from denying the agency is estoppel. This legal principle is based on the idea that a party is prohibited from taking a position that contradicts a previous statement or behavior when another party has relied on that previous action or statement. In the context of agency relationships, if a broker allows a person to act as their agent—knowingly or unknowingly—and the third parties rely on this person's authority to act on the broker's behalf, the broker is "estopped" from denying the agency relationship later on.

This ensures consistency and fairness in dealings and protects third parties who may have acted in good faith based on the broker's representation. Essentially, estoppel serves to uphold the trust placed in the agency by others and reinforces the broker's responsibility regarding the actions of their agents.

The other concepts do not apply in this scenario: ratification refers to the formal approval of an agent's actions after they’ve occurred; subrogation deals with the substitution of one party for another in terms of rights or claims, often in insurance contexts; and reformation involves changing a contract to reflect what parties intended but didn’t accurately capture in writing. These concepts do not directly relate to the limitation on denying an agency relationship as estoppel does