What assists in the brake system by decreasing the driver's physical effort?

The correct response highlights the role of the power booster in a brake system. A power booster, often referred to as a brake booster, enhances the braking process by increasing the force applied to the brake pedal. This device uses vacuum pressure generated from the engine (when equipped with a vacuum booster) or hydraulic pressure to amplify the driver's input, making it easier for them to apply brakes without requiring excessive physical effort.

This is especially beneficial in larger or heavier vehicles where manual braking alone would be difficult or inefficient. By reducing the driver's physical strain, the power booster effectively enhances safety and control, ensuring that even in emergency situations, the driver can exert sufficient force on the brakes with minimal effort.

While other components like the vacuum assist booster might seem similar in function, it is more specific to the type of booster that relies on vacuum pressure, which is a subset of power boosters. The return spring's role is more about returning components to their original positions rather than assisting in force application, and the lever is a mechanical component that does not provide the same amplification effect as the power boosting mechanism.