For a coupling device other than a fifth wheel mechanism, it must have a:

Answer: C — Auxiliary safety chain

A coupling device is used to connect a trailing unit to a powered unit, such as a trailer to a truck. For all coupling devices other than a fifth wheel mechanism, it is required to have an additional safety measure in place. This is why choice A, a special permit, is incorrect. Permits are not safety devices meant to protect against potential accidents. Choice B, a locking pin mechanism, may be one type of auxiliary safety measure but it is not the only option. Choice D, a secondary coupling device, is not a specific safety measure and may simply refer to another primary coupling device rather than an additional safety feature. The correct answer, choice C, a safety chain, is used as a backup in case the primary coupling device fails. It helps prevent the trailing unit from separating from the powered unit, reducing the risk of accidents on the road.