What cloud type is characterized by their flat base and extensive horizontal development?

The correct answer identifies stratus clouds, which are characterized by their flat bases and extensive horizontal development. Stratus clouds typically form in uniform layers that cover the sky, resembling fog at higher altitudes. These clouds often lead to overcast conditions, and light precipitation may occur, such as drizzle or light rain. The flat nature of stratus clouds results from rising air that spreads out more horizontally instead of building up vertically, which is how clouds like cumulus or cumulonimbus develop.

In contrast, cumulus clouds are associated with vertical growth and appear puffy and white, often signaling fair weather. Cumulonimbus clouds are towering and vertically developed, often related to thunderstorms and severe weather. Cirrus clouds, on the other hand, are high, thin, wispy clouds composed of ice crystals and do not have the flat bases seen in stratus clouds. Thus, the defining characteristic of flat bases and horizontal spread makes stratus clouds the correct identification in this context.