If fog is greater at the foot of a hill and less at the summit what does it indicate?

Correct answer: Accumulation of cold moist air in lower part

The correct answer is A: cold, moist air has accumulated in the lower part. During suitable conditions, especially at night, cooled air becomes denser and can flow or settle into hollows and low-lying areas. If this air contains enough moisture and reaches saturation, condensation produces many tiny suspended droplets, forming fog. The hilltop may remain above the cold-air pool or may experience more mixing and wind, so fog can be thinner there. B is wrong because water vapour is not always greater at a summit; the observation points instead to local relief and cold-air pooling. C is wrong because air is present at the foot, even if it is calm. D is wrong because a mountain does not magically destroy fog. Memory cue: cold air sinks and collects; low places often become foggy.