What role does the APU play during pre-flight checklists?

The auxiliary power unit (APU) plays a crucial role in ensuring that all systems are operational during pre-flight checklists. This small but vital engine provides necessary power to the aircraft's electrical systems, allowing crew members to conduct critical pre-flight checks without needing assistance from ground power sources. The APU enables the operation of cockpit instruments, lighting, and environmental controls, which are essential for verifying the functionality of various systems before takeoff.

In this context, the APU allows for the testing of communication systems, navigation equipment, and other essential functions that ensure the aircraft is ready for flight. By providing a reliable power source for these checks, the APU helps confirm that all systems are performing properly, contributing to the overall safety and readiness of the aircraft.

The other options, while relevant to other aspects of aircraft operations, do not accurately describe the specific role of the APU during pre-flight procedures. For instance, the APU does not provide additional fuel or increase aircraft weight, nor is its primary function related to cargo loading. Its primary purpose during pre-flight is to ensure that the aircraft's systems are functional and ready for flight.