What is the electrical source that supplies APU during start?

The correct choice indicates that during the start of the Auxiliary Power Unit (APU), the electrical source is supplied by Battery 1 or the Ground Power Unit (GPU). This is essential because the APU requires a reliable and robust electrical source to initiate the starting sequence. The Battery 1 provides direct power which is necessary for the ignition and starting processes.

When using the GPU, it serves as an external power source that can easily provide the required electrical load to start the APU without depleting the aircraft's batteries, which is particularly useful during ground operations when the engines are not running. The provision of electrical power from these sources ensures that the APU can operate independently of the main engines, enabling power to systems while on the ground and during the initial phases of flight preparation.

Other options describe systems that are not used as the primary electrical source during the APU start process, leading to the conclusion that Battery 1 or GPU is indeed the appropriate answer.