Which part forces fluid to the master cylinder when brakes are applied?

The correct answer is the brake booster. The brake booster is a device that amplifies the force applied on the brake pedal, making it easier to apply the brakes. When the driver presses the brake pedal, the brake booster uses vacuum pressure or hydraulic force to assist in pushing the brake fluid from the master cylinder into the brake lines, thus applying the brakes more effectively.

In contrast, while a solenoid valve is involved in specific electronic control systems, it does not play a role in mechanically forcing fluid to the master cylinder. The caliper is part of the braking system that houses the brake pads and applies pressure to the brake rotor but does not directly influence fluid movement from the master cylinder. A piston, which is often part of the brake caliper or master cylinder, works by moving under pressure to apply the brakes, but it is not the component that directly forces fluid into the master cylinder when brakes are applied. The brake booster is specifically designed for this purpose, making it the correct choice.