If fog is denser on a lower road section which topographic cause is likely?

Correct answer: Accumulation of cold moist air in the lower part

Direct answer: A is correct. In a valley, hollow, or other low-lying road section, gravity can cause relatively cold, dense air to drain downward and collect. If that air contains enough moisture, night-time cooling can bring it to saturation. Water vapour then condenses into many tiny droplets, producing fog. The depression may also be sheltered from wind, so mixing is weak and the droplets remain concentrated, making visibility poorer than on nearby higher ground. A correctly combines low relief, cold-air pooling, moisture, and weak mixing. B is wrong because air above does not become literally zero. C is not a normal cause of fog; melting of a road does not explain suspended droplets. D is unrelated and physically incorrect under ordinary conditions. Memory cue: cold, moist air settles in hollows; hollows are often fog-prone. The cause is likely, not automatic, because moisture and suitable cooling are also needed.