What are the greatest threats to an aircraft operating in the vicinity of thunderstorms?

The greatest threats to an aircraft operating near thunderstorms primarily include hail and turbulence, making this the most accurate choice.

Hail can cause significant damage to an aircraft’s exterior, including the windshield, wings, and engine components. Thunderstorms can produce severe hail, which is hazardous for any aircraft flying in or near the storm. Turbulence associated with thunderstorms is exceptionally dangerous because it can lead to sudden and severe changes in altitude and airspeed, posing a risk to the aircraft's control and structural integrity. This turbulence can arise from downdrafts, gust fronts, or microbursts, all of which are phenomena commonly associated with thunderstorms.

While thunder and heavy rain (the first option) and wind shear and low clouds (the second option) are indeed threats, they tend to represent indirect or less severe dangers compared to the immediate risks posed by hail and the violent turbulence that can occur. Precipitation static and fog (the fourth option) also don't encapsulate the primary dangers, as these factors tend to hinder visibility rather than present a direct risk to the aircraft's structural safety or operational performance during turbulent storm conditions. Thus, the specific threats posed by hail and turbulence define the answer as the most critical concerning aircraft safety in thunderstorms.