Which type of cloud is known as "cumulonimbus"?

Cumulonimbus clouds are indeed known as heavy thunderclouds. This type of cloud is characterized by its towering vertical structure, often reaching high into the atmosphere, and it is associated with severe weather conditions such as thunderstorms, heavy rain, hail, and sometimes even tornadoes. Cumulonimbus clouds have a distinct anvil-shaped top and can develop from cumulus clouds. Their ability to produce intense weather events makes them significant in meteorology and aviation, as pilots must navigate carefully around these clouds due to their associated turbulence and lightning.

The other types of clouds mentioned do not carry the same characteristics as cumulonimbus. Thin stratus clouds are typically flat and layered, lacking the vertical height and intensity of cumulonimbus. Layered cloud formations generally refer to clouds that do not exhibit the towering structure essential to heavy thunderstorm development. Flat low-level clouds, while they can cover the sky, do not possess the dynamic weather-producing capabilities associated with cumulonimbus clouds.