Which situation produces the flat base of cumulus clouds?

The correct choice highlights that the flat base of cumulus clouds is formed when rising air reaches the dew-point temperature. This process initiates the condensation of water vapor into tiny water droplets, leading to the visible formation of clouds. As warm, moist air rises, it expands and cools. Once the air cools to the dew point, the moisture it contains begins to condense, creating a cloud. The flat base occurs at this level where the condensation begins, as all parcels of air at this height reach the same temperature and pressure, creating a relatively uniform base.

Other situations may contribute to cloud formation but do not fundamentally explain the specific characteristic of the flat base of cumulus clouds. For instance, while stable atmospheric conditions might suppress vertical growth and cloud formation, they don't specifically describe the mechanism behind the flat base. Cooling of surface air can lead to condensation but does not directly lead to the flat base without ascending air reaching the dew-point, and high humidity levels alone do not guarantee cloud formation or a flat base.