Which of the following is a common indication of an electrical system fault?

Flickering cockpit lights are a common indication of an electrical system fault because they typically suggest an irregular supply of power to the lighting system, which can result from a variety of issues such as poor connections, a failing generator, or other electrical malfunctions. This instability in the lighting system can reflect broader problems within the aircraft's electrical system that may affect other electronic components and instruments.

In contrast, stable cockpit readings, continuous engine performance, and consistent altitude readings generally suggest that the electrical system is functioning properly, as these stable indicators are critical for reliable operation and safety in flight.