How can saturation for fog formation be achieved?

Correct answer: By cooling air or adding water vapour to it

Direct answer: A is correct. Saturation means that air contains as much water vapour as it can hold at its current temperature; in simple terms, relative humidity reaches about 100 percent. There are two basic routes. First, cool moist air until its temperature reaches the dew point; the cooler air can hold less vapour, so condensation begins. Second, add water vapour to air until its existing temperature can no longer accommodate the added moisture. Near the ground, either process can help tiny droplets form and produce fog, provided suitable condensation nuclei are present. A includes both routes. B is wrong because removing moisture moves air away from saturation. C is wrong because a dry surface does not guarantee saturated air. D is wrong because removing nuclei makes droplet formation more difficult rather than causing saturation. Memory cue: fog needs saturation—cool the air or add moisture.