What is the function of the emergency air line?

The emergency air line serves a critical safety function in commercial vehicle operation. Its primary purpose is to engage the trailer brakes in the event of a loss of air pressure. This is particularly important because if the air pressure drops below a certain threshold, it indicates that the brake system may not function properly, posing a serious hazard. By using the emergency air line, the trailer's brakes will automatically engage, thereby preventing the trailer from rolling away and enhancing overall safety during operation.

In contrast to this, the other options address functions that do not correspond to the role of the emergency air line. Enhancing fuel efficiency, controlling the trailer's steering, and accelerating on inclines are not functions associated with the emergency air line. Rather, those aspects relate to different vehicle systems that manage fuel consumption, vehicle direction, and power, respectively.