How is a vehicle supported while being towed by a truck tractor?

A vehicle being towed by a truck tractor is supported primarily by the kingpin. The kingpin is a crucial component that connects the tractor and the trailer. It acts as a pivot point, allowing for secure attachment and providing stability during towing. This connection is essential for transferring the weight of the trailer onto the tractor, ensuring that the trailer remains stable while in motion.

The design of the kingpin allows for the towing vehicle to manage the load effectively and maneuver with precision. This system provides both structural integrity and safety when transporting loads, as it helps to maintain the correct alignment and balance while driving.

Regarding the other options, a front axle, trailer hitch, or rear end of the truck do not play the same critical role in the connection and support of the towed vehicle as the kingpin does, making the kingpin the correct choice.