What cloud type is most closely associated with thunderstorms?

Cumulonimbus is the cloud type most closely associated with thunderstorms.

A cumulonimbus cloud is a tall, dense cloud produced when warm, moist air rises strongly through the atmosphere. Its vertical development can extend from near the surface into the upper troposphere. The cloud often has a characteristic anvil-shaped top when winds spread ice crystals outward near the tropopause.

Cumulonimbus clouds can bring lightning, thunder, heavy rain, hail, strong gusts, and sometimes tornadoes. Not every cumulonimbus produces every hazard, and visible cloud appearance alone cannot determine the exact severity of a storm. Cumulus clouds are smaller fair-weather clouds, while stratus clouds form broad layers; neither name specifically identifies a thunderstorm cloud.