Identify the type of cloud that is low, dense, and appears pillowy with a flat grey base.

The cloud described as low, dense, and appearing pillowy with a flat grey base is best identified as stratocumulus. Stratocumulus clouds typically form in low layers and have a lumpy or rounded appearance, often resembling large cotton balls. Their flat grey base is characteristic of these clouds, which may cover the sky but are usually not thick enough to block out sunlight completely.

Cumulus clouds, while also fluffy and white, are generally puffy with a well-defined structure and do not usually present a flat grey base. Instead, they typically have a more distinct vertical development. Cumulonimbus clouds are towering and associated with thunderstorms; they are much taller and denser than the clouds described. Stratus clouds cover the sky in a uniform layer but do not have the pillowy appearance attributed to stratocumulus. Thus, the correct identification of the cloud type as stratocumulus aligns with the description of its characteristics.