While you check if the rear trailer is securely coupled to the front tractor, the landing gear should be __________________.

A slightly raised.

explanation

Coupling Tractor-Semitrailers [...]Step 11: Check the Connection for Security
• Raise the trailer landing gear slightly off the ground.
• Pull the tractor gently forward while the trailer brakes are still locked to check that the trailer is locked onto the tractor. [...] [Commercial Driver Handbook]