What type of cloud is characterized by its fluffy, rounded appearance?

The type of cloud characterized by its fluffy, rounded appearance is cumulus. Cumulus clouds are typically seen as white, puffy formations that look like cotton balls and often indicate fair weather. They are formed when warm, moist air rises and cools, causing the water vapor to condense into visible droplet formations. This rising air can create the distinctive, towering structure in larger cumulus formations, making them appear particularly fluffy.

In contrast, cumulonimbus clouds, although also related to the cumulus type, are known for their towering and vertically developed structure and are associated with thunderstorms and severe weather. Stratus clouds are generally flat and layered, covering the sky in a uniform gray sheet, which does not have the fluffy texture of cumulus clouds. Nimbus refers to clouds that produce precipitation and does not specifically describe a cloud type like cumulus does. Therefore, cumulus clouds distinctly stand out due to their texture and shape, making them the correct answer.