Frost is defined as ice crystal deposits formed by sublimation when temperature and dewpoint are below freezing.

• A. Ice crystal deposits formed by sublimation when temperature and dewpoint are below freezing.
• B. Liquid water droplets freeze on contact with the air.
• C. Water vapor condensing into liquid droplets on surfaces.
• D. Frozen condensation from high humidity on surfaces.

Answer: (C)

Frost forms when water vapor in the air deposits directly as ice crystals onto a surface that is at or below freezing. This is a deposition process — gas turning directly into a solid — not sublimation, which is solid turning into a gas. When the air’s dew point is below freezing, the vapor doesn’t condense into liquid water; instead, it changes straight to ice on the surface, creating frost.

The description that matches frost describes ice crystals forming by deposition from water vapor, given the surface temperature and dew point are below freezing. The other descriptions describe liquid condensation (dew) or freezing of liquid droplets, which are not frost.