What must a coupling device, other than a fifth wheel mechanism, have?

Answer: C — An auxiliary safety chain

A fifth wheel mechanism can cause strain on the coupling device due to heavy loads on long rides; therefore, the National Highway Traffic Safety Administration has issued certain requirements for coupling devices such as the need for an auxiliary safety chain to prevent any detachment in case of unexpected events, like a sudden stop or swerve. An automatic lock system and a double coupling feature do not provide the necessary security in case of unexpected events. A manual release system also does not serve as a reliable safety measure, as it requires manual intervention which may not be possible in certain situations. Hence, the only necessary feature for a coupling device, other than a fifth wheel mechanism, is an auxiliary safety chain.