Rainfall occurs because as solar heating occurs, warm and moist air rises which fuels storm formation. This is best described by option D.
According to the process of convection, warm air rises and cool air sinks. This is applicable to rainfall because warm and moist air rises and is absorbed by the clouds in a process called condensation or reverse vaporization. As more air rises, more clouds accumulate rain and the rain eventually gets to heavy and drops in the form of a liquid.