Final answer:
To ensure the trailer brakes are working after supplying air, apply and release the trailer brakes while listening for the sound of engagement. This safety check, known as a brake check, verifies the functionality of the brake system and is necessary before starting a trip.
Step-by-step explanation:
After you supply air to the trailer, making sure the air lines are not crossed and trailer brakes are working is crucial for safety. This is done by: C) Applying and releasing the trailer brakes and listening for the sound of brake engagement. This process is known as a brake check and it is important to ensure that the brake system is functioning properly before starting your journey. When you apply the trailer brakes and listen, you are making sure that the brakes engage and disengage correctly without delays or unusual noises. If the brakes do not function correctly, it could indicate an issue such as crossed air lines, which would require immediate attention before driving the vehicle.
It is not sufficient to rely on lifting the brake pedal or watching mirrors to check if trailer lights come on (options A and B) as these methods do not accurately diagnose brake functionality. Safe operation requires a direct test of the brake system through auditory confirmation of proper brake engagement.