What visual indicators signify extreme turbulence in thunderstorms?

Frequent lightning and roll clouds are notable visual indicators of extreme turbulence in thunderstorms. Lightning occurs due to the rapid buildup and discharge of electrical energy within a storm, often indicating intense updrafts and downdrafts that characterize severe turbulence. Roll clouds, which are low, horizontal, tube-shaped clouds associated with thunderstorm outflow, suggest the presence of strong wind shear and can indicate significant turbulence nearby as they mark areas where the air is violently rotating.

The presence of both of these indicators often correlates with severe weather conditions and turbulence, making option A the most appropriate choice for identifying extreme turbulence in thunderstorms. The other options do not indicate such severe turbulence conditions directly, as heavy rainfall and low visibility can occur in various weather scenarios without necessarily being indicative of extreme turbulence. Cumulonimbus clouds, while part of thunderstorm development, combined with a clear sky, does not accurately represent turbulence conditions, just as high cirrus clouds with scattered rain are typically associated with less severe weather.