Shear turbulence from a thunderstorm has been encountered:

Shear turbulence is primarily associated with the wind shear effects around thunderstorms, which can extend significant distances from the storm itself. The correct answer indicates that shear turbulence can be encountered up to 20 miles away from a severe thunderstorm. This distance reflects the understanding that severe thunderstorms can produce strong winds and turbulence both at and below cloud level, which can reach outwards as the storm's influence spreads.

The dynamics of thunderstorm activity can lead to turbulent conditions well beyond the storm's immediate vicinity. Studies and operational observations have shown that the turbulence can persist for considerable distances, and meteorological guidelines often suggest providing caution for areas further than just a few miles from the storm.

Other distances provided in the options, while they may seem reasonable, do not align with the known effects of severe thunderstorm-induced turbulence, where 20 miles is generally considered a threshold for potential impacts from shear turbulence. Understanding the range of turbulence effects is vital for flight safety and operational planning in aviation.