What is a key indicator that the brake lights are functioning properly?

Answer: A — They illuminate when the brake pedal is pressed

A key indicator that the brake lights are functioning properly is that they illuminate when the brake pedal is pressed. This means that when a driver steps on the brake pedal, the brake lights should activate, signaling to other drivers and pedestrians that the vehicle is slowing down or stopping. This critical safety feature ensures visibility and communication on the road, which helps prevent rear-end collisions. The other options do not reflect proper functioning of brake lights. For instance, brake lights that stay on continuously indicate a malfunction, such as a stuck switch, which could lead to confusion on the road. Similarly, flashing brake lights are not standard behavior and could mislead other drivers. Lastly, brake lights that remain unlit when the brake pedal is pressed clearly mean that the system is not functioning, which poses a safety hazard.