During an in-flight emergency, how does an APU assist the aircraft?

The correct answer highlights the critical role an Auxiliary Power Unit (APU) plays during in-flight emergencies by providing backup systems. Specifically, the APU supplies necessary electrical power and bleed air when the main engines are compromised or shut down. This functionality is crucial for maintaining essential systems on the aircraft, allowing for operations such as cabin pressurization, air conditioning, and powering flight instruments.

In the context of an emergency, having the APU operational means that crew members can maintain better control over the aircraft systems, decreasing the likelihood of further complications and ensuring that safety measures can still be enacted. The other options do not accurately represent the role of an APU. It does not power the main engines, nor does it directly increase altitude performance or reduce fuel consumption. Instead, it serves as a vital safety net that supports the aircraft systems, ensuring continued operation even when primary power sources are unavailable.