What type of cloud formation is often associated with severe thunderstorms?

Cumulonimbus clouds are particularly significant when discussing severe thunderstorms because they are large, towering clouds that can extend high into the atmosphere. These clouds are characterized by their anvil-shaped tops and are capable of producing heavy rain, lightning, strong winds, hail, and even tornadoes. The vertical development of cumulonimbus clouds is indicative of strong updrafts, which are critical in the formation of severe weather.

In contrast, cumulus clouds are generally puffy and do not usually lead to severe weather. Cirrus clouds, which are high-altitude, wispy clouds, indicate fair weather and are not associated with thunderstorms. Stratus clouds typically form in uniform layers and are more associated with overcast conditions, often leading to light rain but not severe thunderstorms. Thus, cumulonimbus is the correct choice due to its direct association with severe thunderstorm activity.