What must brake lights be activated by?

Brake lights are designed to alert drivers behind a vehicle when the vehicle is decelerating or stopping. These lights must be activated by the service brake, which is the primary braking system used for normal driving conditions. When the driver presses the brake pedal, the service brake engages, causing the brake lights to illuminate. This provides a vital safety signal to other road users that the vehicle is slowing down or preparing to stop, helping to prevent potential rear-end collisions.

In contrast, other options mentioned do not serve the same function. The turn signal is used to indicate a change in direction rather than stopping, while the emergency brake is typically used in specific circumstances, such as when the primary brakes fail. The gas pedal is responsible for accelerating the vehicle, not alerting others when it is stopping.