Which characteristic is typical for cumulonimbus clouds?

Cumulonimbus clouds are known for their vertical development, towering high into the atmosphere, which can reach altitudes of up to 60,000 feet or more. This significant height and density create a wide range of atmospheric phenomena within these clouds, leading to extreme turbulence.

The turbulent conditions arise from the strong updrafts and downdrafts associated with the rapid vertical movement of air within the cloud. These powerful air currents result in instability and can lead to severe weather events such as thunderstorms, heavy precipitation, hail, and even tornadoes.

The other characteristics listed do not typically describe cumulonimbus clouds: they are not associated with very low density, having instead a dense core; they possess a towering and bulky structure rather than being thin and wispy; and they are often found at high altitudes, contradicting the idea of low altitude presence. Thus, extreme turbulence is a defining characteristic of cumulonimbus clouds, making it the correct choice.