Which component must be present in air for dew to form?

Correct answer: Water vapour

Dew forms when water vapour in the air cools and changes into tiny liquid water droplets on a cool surface. The vapour may come from moist air, and the surface often cools during the night. Without water vapour, there would be no water available to condense, so dew could not form. Moisture is therefore an essential component, although moisture alone is not always sufficient; the surface must also cool to the dew point.

Option A is correct because water vapour supplies the material that becomes dew. Iron, coal, and lava are not necessary atmospheric components for ordinary dew formation. A useful distinction is that dew is liquid water deposited on a surface, whereas frost forms when water deposits as ice because the surface temperature is at or below freezing. Thus, the question asks for the necessary moisture source, not every condition required.