What kind of lamp indicates that a vehicle's brake is engaged?

The stop lamp, often referred to as the brake light, is designed to illuminate when the vehicle's brakes are engaged. This is a crucial safety feature that communicates to drivers behind the vehicle that it is slowing down or coming to a stop. When the brake pedal is pressed, the stop lamp activates, typically illuminating in red, making it clear to other road users that the vehicle is not moving at its previous speed.

This function is vital for preventing collisions and ensuring the safe flow of traffic, particularly in situations where a sudden stop might occur. The other options do not serve this purpose; for instance, the tail lamp is used for general visibility of a vehicle from the back, the headlamp provides illumination for driving at night, and the indicator lamp signals intended turns or lane changes. Thus, the stop lamp is the specific indicator associated with brake engagement, making it the correct choice.