Correct option is (c) radiation
Cloudless nights are colder because the nights that have clouds provide a blanket for the earth and trap some of the hot day air where as the cloudless nights have no protection so all the hot air rises up into the sky. radiation is a process in which energetic particles or energetic waves travel through vacuum, or through matter-containing media that are not required for their propagation. Waves of a massive medium itself, such as water waves or sound waves, are usually not considered to be forms of “radiation” in this sense. By contrast, gravitational waves, which are waves of space-time itself, qualify as a type of radiation.
