The area of influence of thunderstorms regarding turbulence is:

The area of influence of thunderstorms concerning turbulence extends below and horizontally from the thunderstorm. This is primarily due to the downdrafts and outflow boundaries produced by thunderstorms, which can create significant turbulence well outside the storm's immediate vertical area.

When a thunderstorm develops, it generates strong updrafts and downdrafts that can affect the turbulence levels in the surrounding air. The turbulence is not limited to just the storm itself; rather, it can extend downwards as the downdrafts push air towards the surface, and horizontally as the outflow from the storm spreads out.

This turbulence can extend for many miles away from the thunderstorm, making it important for pilots and dispatchers to be aware of these conditions. The other options fail to fully account for the turbulence generated below the thunderstorm and in the surrounding horizontal area, which is crucial for the safety of aircraft operating nearby.