Which part of the drum brake assembly contains brake fluid?

The wheel cylinder is the correct choice because it is a crucial component within the drum brake assembly that contains brake fluid. When the brake pedal is pressed, brake fluid from the master cylinder is sent through the brake lines to the wheel cylinder. This fluid is responsible for forcing the pistons in the wheel cylinder outward, which in turn pushes the brake shoes against the inside of the drum, creating the friction necessary to slow down or stop the vehicle.

The design and function of the wheel cylinder are central to the operation of drum brakes, as it directly translates hydraulic pressure into mechanical work. The other components mentioned in the question do not contain brake fluid; the master cylinder stores and supplies brake fluid but does not reside within the wheel assembly itself, while the anchor pin and the shoes are structural components that facilitate the braking process but are not involved in holding or transferring brake fluid.