What characterizes cumulus clouds?

Cumulus clouds are characterized as big, lumpy, billowy clouds that typically indicate instability in the atmosphere. They often form in situations where warm air is rising, leading to the development of vertical growth. This rising air cools as it ascends, causing moisture to condense and form the characteristic fluffy appearance of cumulus clouds. The presence of these clouds can signify that convection is occurring, which is directly linked to atmospheric instability.

In contrast to the other options, thin sheets covering the sky are more indicative of stratus clouds, which lack the vertical development and billowy structure of cumulus clouds. Clouds described as big and lumpy while indicating stability would not accurately reflect cumulus clouds, as stability usually corresponds with a lack of vertical growth and a more uniform cloud cover. Lastly, while cumulus clouds can produce precipitation under certain conditions, they are not primarily identified as clouds with rain falling from them; that characteristic is more characteristic of cumulonimbus clouds or nimbostratus clouds, which are associated with more significant weather systems. Thus, the defining features of cumulus clouds strongly align with the characteristics of option C.