What is the maximum altitude at which the APU can provide bleed air?

The correct answer identifies the maximum altitude at which the Auxiliary Power Unit (APU) can effectively supply bleed air. Specifically, the APU is a crucial component used for providing power, air conditioning, and bleed air for engine starting when the engines are not running or during ground operations.

The ability of the APU to generate bleed air is limited by altitude due to decreased atmospheric pressure and density as elevation increases. At approximately 20,000 feet, the APU can still provide sufficient bleed air for systems such as environmental control and engine starting, while maintaining the necessary performance levels. Beyond this altitude, the APU's efficiency and ability to produce bleed air diminish significantly, impacting its functionality.

Therefore, acknowledging the operational parameters of the APU, the choice that states 20,000 ft as the maximum altitude aligns with the technical specifications and performance capabilities of most APUs used in aviation. Hence, this makes it the correct answer to the question regarding the maximum altitude for bleed air provision.