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

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.

I'm not sure but i will try to find out