A broker who represents both the buyer and seller in the same transaction without the knowledge and consent of both parties is practicing:

A is incorrect because dual agency requires the knowledge and consent of both parties, which is explicitly absent in this scenario. Without this consent, the situation cannot be legally classified as dual agency under California law.

C is incorrect because single agency involves representing only one party in a transaction, not both. The scenario clearly states the broker is representing both buyer and seller, which contradicts the definition of single agency.

D is incorrect because transaction brokerage is a specific form of limited representation where the broker facilitates the transaction without representing either party. This doesn't match the scenario where the broker is actively representing both sides.