What does the stop lamp on a vehicle indicate?

The stop lamp on a vehicle is designed to illuminate when the driver applies the brakes. This visual signal is crucial for alerting traffic behind the vehicle that it is slowing down or coming to a stop. When the brake pedal is pressed, the stop lamps activate, providing a clear indication to other drivers that they need to prepare for a potential stop. This function significantly enhances safety on the road by reducing the likelihood of rear-end collisions.

Other options do not accurately describe the primary purpose of the stop lamp. For example, while the signal of slowing speed might be indirectly inferred from the activation of the stop lamp, it does not explicitly convey the action of braking. The stop lamp is not intended as a warning tool for tailgating vehicles or a notification for pedestrians; those situations involve different signals or actions entirely. Therefore, the specific design and function of the stop lamp emphasize its role in indicating that the brakes are engaged, making option B the correct choice.