What is the minimum temperature required to start the APU on the ground?

The correct choice reflects the operational standards for starting the Auxiliary Power Unit (APU) on the ground. For many aircraft, the minimum temperature to safely start the APU is typically around -40°C. This is a crucial figure as it highlights the APU's capability to function in extremely cold weather, which is vital for maintaining operations in a range of environmental conditions.

APUs are designed to provide electrical power and bleed air for engine starting and environmental control when the main engines are not running. Understanding the threshold for temperature is important for ensuring the reliability of the APU in providing necessary support functions, particularly in colder climates. Thus, knowing that -40°C is the accepted minimum helps in planning for safe and efficient aircraft operations, especially in wintery conditions.