If visibility in mist gradually falls and becomes like fog which change should be considered most decisive?

Correct answer: Increase in number or concentration of condensed droplets in air

Correct answer: D. Mist and fog are both made of tiny suspended droplets, but fog generally has a greater concentration of droplets and therefore poorer visibility. As more water vapour condenses, or as existing droplets become more numerous and concentrated in the air, they scatter and partly absorb more light. The observer then sees a shorter distance, and mist can reach fog-like conditions. A is wrong because completely dry air would reduce the possibility of forming droplets. B is wrong because frost is deposition on a cold surface and is not the decisive measure of visibility in the air. C is wrong because river flow does not define the mist–fog change. D is correct because droplet concentration directly controls light scattering and visibility. Memory cue: more suspended droplets mean more scattered light and less visible distance.