A dense mass of water drops on smoke or dust particles in the lower atmosphere layers is referred to as:

Correct — A. Smog is the name for droplets that condense on smoke particles in the lowest layers of the atmosphere.

Water vapour cannot condense on nothing. It needs nuclei, and in a city or an industrial belt smoke supplies them in millions, so the fog that forms is dense, dirty and sits close to the ground.

The deciding word in the stem is smoke. Fog and mist form on dust and salt nuclei too, but only smog is defined by the smoke in it, so option (a) is what the definition describes.

Why the others are wrong

• (b)
  frost — Frost is deposited as ice, not as drops. It forms when a surface cools below freezing and water vapour turns straight into ice crystals on it, so there is no mass of water droplets at any stage.
• (c)
  blizzard — A blizzard is a wind event — a violent snowstorm that drives loose snow along and cuts visibility. It is weather in motion, not condensation on suspended particles.
• (d)
  mist — Mist is the near miss. The droplet physics is the same, but mist is the thinner, lighter form and carries no smoke in its definition, and the stem names smoke.

Condensation near the ground gives a small family of forms, and SSC tests them by definition.

Fog is a cloud whose base sits at or very near the surface: moist air is chilled and its vapour condenses on the tiny particles floating in it.

Mist is the same process, thinner and less obstructive.

Smog is what fog becomes where those nuclei are smoke — the word itself is a blend of smoke and fog. Frost is a different mechanism altogether: vapour turning directly to ice on a chilled surface, with no liquid stage.

Key facts

• Smog is a blend of the words smoke and fog, and describes fog formed where smoke particles act as condensation nuclei.
• Fog is a cloud with its base at or very near the ground.
• Frost forms by deposition, when water vapour changes directly into ice on a surface below freezing point.
• Mist and fog differ in density, not in mechanism, with fog the denser of the two.

Common traps

• Picking mist because the stem says water drops, and reading past the word smoke
• Treating frost as frozen dew, when frost is vapour deposited straight as ice

No directly related past PYQ was found.