Which of the following best describes a 'towering cumulus' cloud?

A towering cumulus cloud is best described as a large, vertically-developed cloud with significant vertical extent. These clouds are the building blocks of thunderstorms and can develop rapidly under the right atmospheric conditions, indicating strong vertical motion and instability within the atmosphere.

Towering cumulus clouds are characterized by their impressive height and thickness, which can lead to the formation of cumulus congestus or cumulonimbus clouds if further development continues. The vertical growth is due to the rising warm air, which creates the characteristic towering appearance.

In contrast, other descriptions do not accurately capture the defining features of towering cumulus clouds. For instance, low, flat clouds are typically associated with stratocumulus rather than towering cumulus. Small, white clouds that indicate fair weather refer to cumulus clouds but do not encompass the vertical development associated with towering cumulus. Dark clouds that obscure sunlight are usually related to more mature storm systems, such as cumulonimbus clouds, rather than describing the towering cumulus stage specifically.