Which condition does not trigger a Sigmet?

The correct choice indicates that general thunderstorm activity does not trigger a Significant Meteorological Information (Sigmet) issuance. Sigmets are issued for hazardous weather phenomena that have significant impacts on flight safety and are beyond the norm of standard meteorological advisories.

Severe icing, turbulence, and visibility reductions due to dust storms are all considered significant hazards that significantly affect aircraft operations. Severe icing can lead to performance degradation and loss of control, turbulence may cause difficulty in handling the aircraft, and visibility less than 3 miles from dust storms can severely impair a pilot's ability to navigate and see other traffic.

In contrast, general thunderstorm activity, while it can be a concern, is typically addressed through other advisory means, such as AIRMETs (Airmen's Meteorological Information) or specific weather advisories that detail thunderstorm activity without qualifying it as a significant hazard like severe thunderstorms would. Sigmets are specifically for hazardous conditions that are either extreme or present widespread conditions, whereas general thunderstorm activity may not meet the criteria for a Sigmet, as not all thunderstorms pose an immediate threat to flight safety.