If a pilot observes a flashing red signal for an aircraft in flight, what should be their action?

When a pilot observes a flashing red signal while in flight, the appropriate and recommended action is to abort the landing and circle. A flashing red signal indicates that the pilot is being warned to stay clear of the airport environment, often because it’s unsafe to land due to various reasons such as possible obstruction, hazardous weather, or other signal indications from air traffic control.

By choosing to circle, the pilot can safely reassess the situation and wait for further instructions or when it is safe to land. This maneuver ensures the safety of the aircraft and maintains adherence to air traffic control's signals, allowing for effective management of air traffic and preventing potential incidents.

The other options do not align with the implications of a flashing red signal. For instance, beginning a descent to land or continuing on the flight path would disregard the warning, while checking instruments thoroughly might be necessary but does not address the urgent need to change the flight approach indicated by the flashing red signal.