How do you supply air to the trailer tanks?

To supply air to the trailer tanks, pushing in the trailer air supply valve is the correct action. This valve is typically located on the dashboard of the vehicle and is used to connect the air supply from the tractor to the trailer. When you push in this valve, it allows the air to flow from the tractor's air system into the trailer's air tanks, which are essential for operating the trailer's brakes and other air-operated systems.

Understanding this function is crucial for safe operation of combination vehicles, as proper air supply ensures that the brakes on the trailer will engage effectively and respond to the driver's commands during operation.

The other options do not facilitate the correct process of supplying air. Pulling out the trailer air supply valve would disconnect the air supply, manually pumping the air lines isn't a standard or effective method for introducing air into the trailer tanks, and checking the air pressure gauge, while important for monitoring the system, does not directly supply air to the trailer.