What does the APU PWR/FUEL button do when on the ground?

Choosing the first option is accurate because when the APU (Auxiliary Power Unit) PWR/FUEL button is engaged on the ground, its primary function is to open the fuel shutoff valve. This action is crucial as it allows fuel to flow to the APU, enabling it to start and operate. The APU is essential on the ground for providing electrical power and pneumatic pressure to the aircraft systems before the engines are started.

Understanding the role of fuel systems is critical in ensuring that the APU has the necessary resources to function properly, which is vital for pre-flight operations. The other options do not correctly describe the function of the APU PWR/FUEL button in this scenario.