Which statement is true regarding ground power connected to the aircraft?

The statement that external power can be used to operate all electrical systems when the engines aren't operating is accurate because it reflects the functionality of ground power supply systems in aircraft. When an aircraft is connected to an external power source, it can provide electricity to operate all necessary electrical systems, including lighting, avionics, and other essential components, without relying on the aircraft's engines. This is particularly useful during pre-flight checks, maintenance, or when the aircraft is grounded, as it ensures that all systems can be tested or utilized without consuming fuel or running the engines, thereby contributing to safety and efficiency.

The other statements do not fully capture the capabilities or intended use of ground power. While it can indeed be used for maintenance, its role is not limited to that function, as it is essential for everyday operations when the engines are not in use. Additionally, ground power can be utilized safely with the engines operational, allowing for simultaneous checks and operations, benefiting overall operational readiness. Lastly, while external power is crucial for starting the engines, that is only one aspect of its broader applications in providing electricity to a variety of systems on the aircraft.