There is to be NO fuel connection between a tractor and __________.

In the context of propane and fuel logistics, having a fuel connection between a tractor and a trailer is not permissible due to safety concerns. It is critical to prevent any potential leaks or unintended ignition sources that could arise from such connections. Propane is highly flammable, and allowing a direct fuel line between a tractor and a trailer could pose severe risks, especially if the trailer carries materials or equipment that could exacerbate a fire.

This safety measure ensures that the risk of an explosion or fire is minimized and maintains the integrity of the fuel system. Each part of the transport system is designed to operate independently, which helps in managing the safety protocols necessary when handling flammable materials.

The other options involve connections or equipment that are either irrelevant in this context or have established safety regulations allowing for limited connections under controlled conditions.