Which of the following is an uplifting force necessary for thunderstorm formation?

The uplifting force necessary for thunderstorm formation is primarily due to heat from the ground. When the sun heats the earth's surface, the air closest to the ground warms up as well. Warm air is less dense than cooler air, causing it to rise. This rising motion of warm air is essential for the development of thunderstorms. As the warm air ascends, it cools and can lead to condensation, forming clouds and potentially leading to thunderstorms.

In terms of the other options, while humidity levels are important for thunderstorms as they contribute to the moisture content of the air, they do not directly cause lifting. Cool air sinking is counterproductive to thunderstorm formation because it tends to suppress upward motion. High pressure systems typically lead to more stable atmospheric conditions, which are not conducive to the development of thunderstorms, as they usually inhibit lifting and promote descending air. Thus, heat from the ground effectively provides the necessary uplifting force to initiate thunderstorm activity.