What conditions are necessary for the formation of cumulonimbus clouds?

Cumulonimbus clouds, commonly associated with thunderstorms and severe weather, form under specific atmospheric conditions. High humidity is essential because it provides the moisture needed for cloud formation. The lifting action can come from various sources, such as convection currents, fronts, or mountains, which raise the warm, moist air to cooler altitudes.

The presence of unstable air is crucial as well; it allows the air to rise rapidly, creating strong updrafts. When warm air rises and cools, the water vapor in it condenses into water droplets, forming clouds. In the case of cumulonimbus clouds, this upward movement of humid air must be vigorous enough to overcome the stability of the atmosphere, leading to significant vertical development and the potential for severe weather phenomena such as lightning, thunder, and heavy precipitation.

Thus, the combination of high humidity, the presence of lifting action, and unstable atmospheric conditions is what is necessary for the formation of cumulonimbus clouds, making this the correct choice.