Question

Which process is the most important for cloud formation?
A. cooling by contact with a cold surface
B. cooling by compression of air
C. cooling by expansion of air
D. cooling by radiation from Earth's surface

Answer 1

The most process for cloud formation is C cooling by expansion of air. The process itself is also called condensation. 
When warm air rises, it expands and cools. Cool air can't hold as much water vapor as warm air, so some of the vapor condenses onto tiny pieces of dust that are floating in the air and forms a tiny droplet around each dust particle. When billions of these droplets come together they become a visible cloud.

Answer 2

Answer: Option (C) is the correct answer.

Explanation:

When moisture present in the air rises and cools by expanding into the atmosphere then this result in vapourization of water.

When this vapour condenses into small water droplets then it results in the formation of clouds.

Thus we can conclude that cooling by expansion of air is a process which is most important for cloud formation.