Correct Answer: (b) Clouds reflect back the Earth's radiation.
Solution:The most suitable conditions for dew formation are cler sky, clam air, high relative humidity and cool and long nights. With higher temperature during the day, the water holding capacity of the air increases, due to which the amount of water vapour (humidity) in the air increases. On the con- trary, when the temperature becomes very low at night, the water-holding capacity of the air decreases due to which more water vapour from the saturated air condenses in the from of drops on the cold surface, which is called 'dew'. But when clouds cover the night sky, they reflect back Earth's radition, keeping the air and surface warm. Due to this the diurnal temperature difference is less and does not form.