What is the function of the emergency line in a trailer?

The emergency line in a trailer serves the critical function of supplying air to the emergency brakes on the trailer. This line is crucial in ensuring that the trailer's brakes can engage if there is a loss of air pressure in the system, which could occur due to a rupture or failure of the primary braking system. The emergency brakes act as a fail-safe, allowing the trailer to stop safely, which is especially important in preventing accidents during an emergency situation. This functionality underscores the importance of the emergency line in a trailer’s air brake system, providing an additional layer of safety for both the truck and trailer when on the road.

The other options, such as supplying air to the normal brakes, controlling the air suspension system, or providing power to trailer lights, do not accurately describe the primary role of the emergency line. Each of those functions is associated with different lines or systems within the truck and trailer setup, emphasizing the specific and critical role of the emergency line in facilitating the proper operation of emergency braking mechanisms.