The formation of a thunderstorm occurs if an unstable airmass convects intensely, forming a cloud that can grow to a maximum altitude of about:

The correct answer reflects the characteristics of cumulonimbus clouds, which are associated with intense convective activities such as thunderstorms. Cumulonimbus clouds are capable of significant vertical growth, often reaching altitudes well into the upper troposphere. Specifically, in conditions of extreme instability, these clouds can grow to impressive heights of around 20 kilometers.

This altitude is notable because it allows for the formation of various weather phenomena associated with thunderstorms, such as heavy precipitation, lightning, and hail. The towering structure of cumulonimbus clouds enables the development of powerful updrafts and downdrafts, which are crucial for maintaining the dynamics of a thunderstorm.

Understanding the nature of these clouds and their potential height helps in recognizing how thunderstorms develop and the associated weather hazards they can produce. Thus, the ability of cumulonimbus clouds to soar to such altitudes is a defining factor in the formation and intensity of thunderstorms.