The correct answer to this question is C, Fog. Although they are often confused with clouds, fog is lower lying. Moisture in the fog comes from bodies of water, such as oceans or lakes. It is formed when the difference between the dew point and temperature is less than 2.5 degrees Celsius. Visibility is reduced to less than 1 kilometer.
There are various types of fog. Radiation fog happens when warm air is produced above cold air. Ground fog is when the layer which is shallow sharply cools at its dew point. Heaviest late at night and early in the morning, fog is extremely dangerous because it reduces visibility while traveling.