What gear should the tractor engine be in after uncoupling from the trailer and inspecting the trailer supports?

After uncoupling from the trailer and conducting an inspection of the trailer supports, the tractor engine should be placed in neutral. This is because being in neutral prevents the vehicle from moving unintentionally while the driver is outside the cab to inspect the trailer or perform any other tasks.

When the tractor is in neutral, it allows for safe maneuvering around the truck and trailer without the risk of rolling or shifting, especially on uneven ground or inclines. This aligns with safety procedures that emphasize maintaining control over the vehicle's movement when performing uncoupling and inspections.

Other options like first gear, reverse, and park do not provide the same level of safety and control during this specific scenario. First gear and reverse may inadvertently engage the drivetrain, risking movement, while putting the vehicle in park does not allow for immediate movement if necessary, and is not a suitable position for inspecting after uncoupling.