Which of the following weather events must be reported on PIREPS?

The correct response is thunderstorms because they represent significant hazards to aviation and can greatly impact flight safety. Pilots and meteorologists rely on Pilot Reports (PIREPs) for real-time information about weather phenomena in the vicinity of flight routes. Thunderstorms, specifically, can produce dangerous conditions such as severe turbulence, wind shear, and lightning, all of which can pose serious risks to aircraft. Reporting these conditions helps other pilots avoid potential hazards and enhances overall flight safety.

While the other choices may pertain to weather phenomena, they do not carry the same degree of urgency and necessity for reporting in PIREPs. Clear skies and stable winds, for example, indicate favorable flying conditions and typically don't require a PIREP. Moderate turbulence is a concern, but it is generally more specific to the conditions that may currently exist rather than a significant weather event like thunderstorms that necessitates widespread communication among pilots.