Which of the following is an indication of an impending stall?

A decrease in airspeed is an indication of an impending stall because as an aircraft approaches its stall speed, the lift being produced by the wings begins to diminish. The stall is characterized by a loss of lift, which happens when the angle of attack exceeds a critical threshold, typically at lower airspeeds. Consequently, when a pilot observes a decrease in airspeed, it signals that they should take corrective action to maintain adequate airflow over the wings and avoid reaching the critical angle of attack.

In normal flight conditions, an increase in airspeed, maintained engine RPM, or level flight attitude would not typically suggest an impending stall, as those scenarios indicate stable or increasing aerodynamic conditions. Therefore, recognizing a decrease in airspeed is crucial for pilots to prevent entering a stall situation, ensuring they maintain control of the aircraft.