Why does fog formation become more likely when dew point and air temperature are very close?

Correct answer: Air can become saturated with little cooling

Direct answer: A is correct. The dew point is the temperature to which air must be cooled, at approximately unchanged moisture content, for saturation to begin. If the air temperature is already very close to its dew point, the temperature-dew-point spread is small. Only a little further cooling—perhaps during the night or near a cool surface—may bring the air to saturation. Once saturated, excess water vapour can condense into tiny suspended droplets, producing fog if the droplets remain near the ground. A correctly links a small spread with high humidity and little required cooling. B is wrong because air close to its dew point is usually moist, not completely dry. C is wrong because water vapour is essential to fog. D is wrong because this relationship does not require zero pressure. A small spread raises the likelihood, but wind, lifting, nuclei, and local temperature still influence the actual result. Memory cue: small dew-point spread means the air is nearly ready to fog.