Final answer:
In the context of commercial truck driving, air is typically supplied to the trailer tanks by pushing in the trailer air supply valve. This valve is part of the air brake system and is necessary for the operation of the trailer's brakes.
Step-by-step explanation:
The question is about the process of supplying air to the trailer tanks within the context of truck operation, specifically related to commercial driving. To supply air to the trailer tanks, one typically needs to either push in or pull out the trailer air supply valve. This valve is part of the tractor-trailer's air brake system. The correct option is generally A) Pushing in the trailer air supply valve, which allows compressed air to flow from the tractor to the trailer's brake system. Alternatively, when the system is designed to be engaged by pulling the valve, it would be option B). Option C) Connecting the service line glad hands is how the air lines are connected between the tractor and trailer, but does not in itself supply air to the tanks, though it is a prerequisite for the system to function.