Final answer:
Rain starts to fall from a cloud when water vapor cools and condenses into water droplets, which aggregate till they are heavy enough to fall as precipitation. The presence of a nucleus for droplets to aggregate around is essential, without which supercooling may prevent rainfall. This process is crucial in the water cycle and the Earth's climate. Option number b is correct.
Step-by-step explanation:
What happens that makes rain start to fall from a cloud involves condensation and precipitation. As the water vapor rises and cools in the atmosphere, it undergoes a phase change into tiny droplets of liquid water through the process known as condensation. These droplets can form clouds. For rain to fall, these water droplets must grow in size and eventually become heavy enough to overcome the updrafts and fall as precipitation, such as rain, snow, sleet, hail, or freezing rain. Most of this precipitation falls into the oceans, which are part of the water cycle.
Supercooling can impact the formation of rain. In clouds, small water droplets need to aggregate around a nucleus to reach a size that allows them to fall. Without nuclei like dust particles, ice crystals, or artificially introduced substances like silver iodide or solid CO₂, the droplets can remain suspended as supercooled liquid. When condensation occurs within a cloud, and the droplets aggregate sufficiently, they can fall to the Earth's surface as precipitation. This process contributes to the overall transfer of heat from the ocean to the atmosphere and is a crucial part of weather systems, including the formation of thunderheads and hurricanes.