What performance aspect is indicated by warning lights in the cockpit during APU operation?

The correct choice highlights that warning lights in the cockpit during APU operation serve as indicators of system failure. These lights are designed to alert the crew to any malfunctions or issues that could compromise safe operation. When the APU is not functioning properly, the warning lights activate to signal a problem that requires immediate attention. This is crucial for operational safety and ensures that the crew can take appropriate measures to address any failures before proceeding with mission-critical tasks.

While fuel consumption and electrical deficiencies are important aspects of APU operation, warning lights specifically focus on alerting the crew to system failures that could impact the APU's reliability and performance. Normal operation would not trigger warning lights, as these would indicate that all systems are functioning as intended without any deviations or concerns. Understanding this helps crew members quickly diagnose and respond to potential issues during APU operation.