Which condition is most favourable for fog?

Correct answer: Cooling of moist air

Correct answer: B, cooling of moist air. Fog forms when air close to the ground contains enough water vapour and cools to its dew point. At that temperature, the air becomes saturated and some water vapour condenses into very small droplets. These droplets remain suspended near the surface and reduce visibility. A is incorrect because very dry air contains little water vapour, while very hot air is generally less favourable for condensation unless it later cools. C is incorrect because melting rocks is a geological or volcanic process, not the atmospheric process that produces fog. D is incorrect because an underground stream is not the direct requirement for fog; the essential conditions are moisture, cooling, and suitable condensation nuclei. Fog may be encouraged by calm conditions because the droplets remain near the ground, but the central idea is moist air cooling. Memory cue: moisture plus cooling brings air to the dew point and can produce fog.