Which measure is most useful while differentiating dense fog and light mist?

Correct answer: Visibility distance

Direct answer: C, visibility distance. Fog and mist are both made of tiny water droplets suspended near the ground, but they differ mainly in how strongly they obstruct vision. Dense fog contains enough droplets or has enough optical thickness to reduce the distance at which objects can be seen. Light mist usually permits comparatively greater visibility. Therefore, measuring or observing visibility distance is the most useful practical basis for distinguishing them. A is wrong because sea-wave height describes ocean conditions, not atmospheric visibility. B is wrong because rock hardness has no role in classifying fog or mist. D is wrong because soil colour does not measure suspended droplets or visual obstruction. Visibility can also be affected by darkness, smoke, dust, and terrain, so observations should consider the wider weather situation; nevertheless, for this school-level comparison, visibility distance is the key measure. Memory cue: the thicker the suspended droplets, the shorter the visible distance.