What conditions are necessary for the formation of cumulonimbus clouds?

The formation of cumulonimbus clouds is primarily associated with a lifting action combined with unstable, moist air. When air rises, it expands and cools. If the air being lifted is unstable, it means that it will continue to rise vigorously after reaching its lifting condition. This instability, when coupled with sufficient moisture, leads to the formation of towering cumulonimbus clouds, which can produce severe weather phenomena such as thunderstorms, heavy rain, and even tornadoes.

Moist air is crucial because as it rises and cools, the water vapor condenses into tiny water droplets, forming clouds. The lifting action can be provided by various factors such as convection currents, frontal activity, or orographic lift. This combination of unstable air and moisture is essential for the development of these clouds, making that choice the correct answer.