Which local condition is most decisive for dew formation?

Correct answer: The surface temperature falls to the dew point but remains above freezing

Answer: A. Dew forms when moist air immediately above a surface cools to its dew point, the temperature at which it becomes saturated and water begins to condense. For ordinary liquid dew, the surface remains above the freezing point, so the condensed water stays liquid rather than becoming frost. Option A is correct because it gives both essential clues: cooling to the dew point and a temperature above freezing. Option B is wrong because high daytime temperature generally prevents condensation and promotes evaporation. Option C is wrong because condensation nuclei help water vapour form droplets; removing them would not favour normal dew formation. Option D is wrong because some water vapour is necessary. If the surface falls below freezing, the expected deposit is frost rather than ordinary dew. Memory cue: dew point starts condensation; freezing point decides dew versus frost.