Liquefaction weather conditions

A, the hail is formed by solidification, which does not meet the meaning of the question;

B, light snow is a small ice crystal formed by condensation of water vapor in the air, which does not meet the meaning of the question;

C, fog is a small water droplet formed by cold liquefaction of water vapor in the air, which conforms to the meaning of the question;

D, frost is a small ice crystal formed by condensation of water vapor in the air, which does not meet the meaning of the question;

So choose C.