Final answer:
The ideal action before reversing a trailer when no observer is available is for the driver to exit the vehicle and check surroundings to ensure safety.
Step-by-step explanation:
Before reversing a trailer without an observer, the driver should exit the vehicle to check surroundings. This method ensures safety as it provides a direct view of any obstacles, pedestrians, or other vehicles that may be in the blind spot of the mirrors or cameras. While side mirrors can aid in visibility, they have limitations. Using a rear-view camera is helpful, but it does not replace the need for a physical check, as cameras can have blind spots or provide a distorted view of distances. Honking the horn can alert others to the vehicle's movements but does not provide visual confirmation of a safe path. Exiting the vehicle allows the driver to perform a comprehensive safety check before maneuvering.