Which action is essential before driving in reverse?

Looking out the rear window is essential before driving in reverse because it provides the driver with a clear view of the area directly behind the vehicle. This action helps to identify any obstacles, pedestrians, or vehicles that may be in the path of the car, ensuring that the driver can safely maneuver the vehicle without causing any accidents. Checking mirrors can be useful, but mirrors only provide a limited view of what is directly behind, and they may not account for blind spots. Sounding the horn could alert others but does not contribute to the safety of the maneuver itself. Making a sharp turn while in reverse is not advisable, as it can lead to a loss of control or failure to see what is behind the vehicle. Therefore, looking out the rear window is the most effective way to ensure safety while reversing.