What additional safety feature is required when coupling a vehicle?

The requirement for an auxiliary safety chain when coupling a vehicle serves as an essential safety measure to ensure that the connection remains secure during operation. In the event that the primary coupling mechanism fails, the auxiliary safety chain acts as a backup, preventing the trailer from disconnecting completely from the towing vehicle. This redundancy is crucial for maintaining control and avoiding accidents that can occur if a trailer detaches while on the road.

The other options, while important for vehicle safety and function, do not serve the specific purpose of providing a secondary connection between the vehicle and trailer. The fifth wheel is a coupling mechanism, brake lines are vital for reducing speed and stopping, and emergency lights are necessary for signaling, but none of these options act as a supplemental safety feature during coupling like an auxiliary safety chain does.