Final answer:
When unsure about what's behind the vehicle while backing, one should stop, get out, and check the path to avoid any potential hazards.
Step-by-step explanation:
If you are unsure what may be behind your vehicle while backing, the safest action to take is A. Stop, get out of your vehicle, and check your path. This ensures that you are aware of your surroundings and prevents potential accidents. Mirrors have blind spots and cannot show you every hazard, and while sticking your head out might give you a better view, it is still limited. Therefore, physically checking the area behind your vehicle is the most reliable method.