Which of the following correctly describes cloud formation?

Clouds form when the atmospheric temperature reaches the dew point because this is when the air becomes saturated with moisture. At this temperature, the water vapor in the air condenses into tiny water droplets or ice crystals, depending on the temperature. This process is essential for cloud formation, as it indicates that the air can hold no more moisture in vapor form, and therefore, condensation occurs, leading to the visible clouds we see in the sky.

The other options do not accurately describe the process of cloud formation. For instance, clouds do not necessarily form at ground level since they can develop at various altitudes in the atmosphere. Additionally, humidity must be at least above zero for clouds to form, as the presence of water vapor is crucial—thus humidity cannot be zero. Lastly, while clouds can be found at higher altitudes, they can also form at lower elevations, so there is no requirement for cloud formation to occur exclusively above 10 kilometers. Therefore, the correct choice highlights the critical role of reaching the dew point in the formation of clouds.