Which type of cloud is characterized by a large, puffy appearance with a flat bottom?

The cloud type characterized by a large, puffy appearance with a flat bottom is stratocumulus. Stratocumulus clouds typically appear as low, lumpy, white or gray clouds that often blanket the sky but can also present in patches. Their appearance resembles a collection of rounded masses or rolls. The flat bottom of these clouds is a distinct feature, indicating that they form at a low altitude and are composed mainly of water droplets.

Cumulonimbus clouds, while also appearing puffy, are known for their towering structure and the potential to produce severe weather, making them quite different in appearance and characteristics. Stratus clouds, on the other hand, are more uniform and layered, leading to a gray overcast sky without the puffy formations found in stratocumulus clouds. Cirrus clouds are high, wispy clouds that do not exhibit the puffy shape or flat bottom associated with stratocumulus. Therefore, the defining features of stratocumulus clouds clearly align with the question's description.