What is the purpose of the emergency air line in a trailer's air system?

The correct answer is indeed focused on the emergency air line's primary function within a trailer's air system. The emergency air line is designed to supply air to the trailer's braking system in the event of a failure in the service line. This is a crucial safety feature, as it ensures that the trailer remains capable of braking even if there is a malfunction in the service air supply. By maintaining brake functionality, the emergency air line helps prevent accidents and provides a critical safeguard for both the driver and other road users.

The other options do not relate to the primary function of the emergency air line. Controlling turning signals or powering the trailer's radio are not functions associated with the trailer's air system, and adjustments to the suspension are typically managed through other systems that do not involve the emergency air line.