What should be done before backing up a truck with a digger derrick?

Using a rearview mirror and having an observer signal is crucial for safely backing up a truck with a digger derrick. This practice enhances situational awareness and helps to detect any obstacles or hazards that may not be visible to the driver. The rearview mirror provides a view behind the vehicle, but it may not cover all blind spots. Therefore, having an observer assists in ensuring that the area is clear and safe, reducing the risk of accidents or collisions during the backing process.

When it comes to the other options, while ensuring the loading area is clear is important, it does not encompass the proactive steps needed while reversing. Checking the fuel level and making sure tires are inflated are essential for the vehicle's overall safety and functionality but are not directly related to the act of backing up the truck safely. Thus, the most comprehensive approach to ensure safety while backing up involves using visual aids and additional personnel for signals and assistance.