Final answer:
APU bleed air feeds into the engine bleed duct, providing pneumatic and electrical power when the aircraft's engines are not running.
Step-by-step explanation:
The Auxiliary Power Unit (APU) bleed air typically feeds into the c) Engine bleed duct. The APU is used to provide pneumatic and electrical power while the aircraft's engines are not running, such as when the aircraft is on the ground. The APU can also supplement power during certain in-flight conditions. The bleed air from the APU is used to start the engines and to provide air conditioning and pressurization for the aircraft cabin when the engines are not running.