Fog is a cloud that has

a. a small vertical extent.

b. no ability to produce rain.

c. particles made of ice crystals.

d. its base at or near the ground

Fog is a cloud that has its base at or near the ground. The answer is letter D. Fog results when atmospheric water vapor condenses to the extent that the new forms, water droplets or ice crystals, become visible. Saturation of the air and sufficient condensation nuclei are ordinarily prerequisites for the formation of fog. One of the great hazards associated with fog is a lack of visibility.