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Dew forms by the condensation of water vapour(which is a gas) into water droplets over thin exposed non-metallic objects.This condensation is brought about by cooling the air which contains enough water vapour.'A' is the nearest answer.

Dew forms when a surface cools (through loss of infrared radiation) to a temperature which is colder than the dewpoint of the air next to the surface. Dew is made of liquid water that has condensed from some of the water vapor contained in the air. If the layer of air next to the ground were actually cooling, then fog would form. Instead, it is just the surface (for instance, grass) that is cooling, and a very thin layer of air next to the grass deposits some of its water vapor in the form of liquid water on the grass

A. Dew is water vapor (gas) condensing down (cooling) to a liquid state.