Fog is defined as:

• A. A weather condition where the sky is completely clear.
• B. Cloud consisting of numerous minute water droplets and based at the surface; droplets are small enough to be suspended in the earth's atmosphere indefinitely.
• C. A fog that falls to the ground as drizzle.
• D. A type of haze without moisture.

Answer: (B)

Fog is a cloud that forms at the Earth's surface. It consists of a large number of tiny water droplets suspended in the air, with the base at or very near the ground. These droplets are small enough to be kept aloft by air currents, which is why visibility is greatly reduced. The other descriptions don’t fit: fog isn’t clear sky; haze without moisture isn’t fog; and while fog can produce drizzle in some cases, the defining idea is that it’s a surface-based cloud of tiny droplets suspended in the air.