What must a vehicle's brake lights do during an inspection?

The requirement for a vehicle's brake lights during an inspection is that they must illuminate brightly and function properly when the brake pedal is pressed. This is essential for vehicle safety, as brake lights signal to other drivers that the vehicle is slowing down or stopping, thus helping to prevent rear-end collisions.

By lighting up when the brake pedal is engaged, the brake lights serve their primary purpose of alerting drivers behind the vehicle, ensuring safer road conditions. This feature is a crucial aspect of vehicle compliance with safety regulations.

The other options do not fulfill the necessary criteria for proper brake light function. For example, having brake lights that stay on continuously would confuse other drivers and could lead to accidents. Requiring that they only illuminate when the headlights are on ignores the need for visibility under all driving conditions. Flashing lights while in motion could mislead other drivers and might not effectively communicate stopping intent. Thus, the correct specification is that the brake lights should illuminate brightly and function correctly in response to the application of the brake pedal.