What can be used to check the proper operation of the stoplight switch in an air brake system?

Answer: B — Activating the brakes and verifying the stop lights illuminate

To ensure the safe operation of an air brake system, it's vital to verify that the stoplight switch functions correctly. Activating the brakes and observing whether the stop lights illuminate directly tests the effectiveness of the switch. When the brake pedal is depressed, the stoplight switch should send a signal to the lights to turn on, indicating to others on the road that the vehicle is slowing or stopping. This is a critical safety feature, as it enhances visibility for other drivers, helping to prevent collisions. The other options do not relate to checking the stoplight switch's functionality. Monitoring engine temperature changes is relevant for engine health but does not provide any information on the brake lights. Listening for unusual sounds when braking can give insights into potential brake issues but does not specifically verify the operation of the stoplight switch. Checking tire pressure is important for overall vehicle safety but is unrelated to the brake system's electrical components. Thus, activating the brakes and verifying the stop lights illuminate is the correct method for checking the proper operation of the stoplight switch in an air brake system.