What type of clouds are associated with unpredictable rain and may also have sleet and hail?

Cumulonimbus clouds are towering clouds that often produce thunderstorms and are specifically associated with unpredictable weather conditions, including heavy rain, sleet, and hail. These clouds can develop rapidly and extend high into the atmosphere, creating an unstable environment conducive to severe weather. The vertical development of cumulonimbus can lead to the formation of significant precipitation as well as other phenomena like lightning and gusty winds.

In contrast, stratus clouds typically form in uniform layers and bring steady, gentle rain rather than the intense and unpredictable storms associated with cumulonimbus. Cirrus clouds, being high-altitude and wispy, indicate fair weather and are not related to precipitation. Nimbus clouds, while they indicate rain, are a less specific classification and do not exclusively refer to the severe weather and mixed forms of precipitation that cumulonimbus clouds can produce. Thus, the association of cumulonimbus clouds with severe weather conditions clarifies why they are the correct answer.