The most likely stage of the water cycle to be occurring on a clear, sunny, hot day is evaporation.
Evaporation occurs when water changes from a liquid to a gas (water vapor) and enters the atmosphere. On a clear, sunny, hot day, the sun's heat causes water on the surface of bodies of water, such as oceans, lakes, and rivers, to evaporate and enter the atmosphere. This process is also called solar radiation because it is driven by the sun's energy.
During evaporation, water molecules absorb energy from the sun, which causes them to move more quickly and break free of their bonds with other water molecules. As the water molecules gain enough energy, they escape into the atmosphere as water vapor. Once in the atmosphere, water vapor can travel long distances and participate in other stages of the water cycle, such as condensation and precipitation.
Overall, a clear, sunny, hot day is ideal for evaporation to occur because it provides the heat needed to drive the evaporation process.