Ringelmann scale' is used to measure density of

1. A

   Smoke

   (a) Smoke. The Ringelmann scale, devised by Maximilian Ringelmann, is a 0–5 chart used to judge the apparent density of smoke plumes from chimneys. Shade 0 is white/clear; shade 5 is black. That smoke-density use is the official key.

2. B

   Polluted water

   (b) Polluted water. Water turbidity or biochemical oxygen demand uses other meters, not Ringelmann cards.

3. C

   Fog

   (c) Fog. Fog density is a meteorological visibility problem, not a Ringelmann reading.

4. D

   Noise

   (d) Noise. Sound is measured in decibels on a sound-level meter, unrelated to Ringelmann shades.

Summary. Official key is (a) Smoke. Ringelmann cards compare chimney smoke with printed greys to grade opacity. They are a classic air-pollution field tool, not a water, fog or noise gauge. Neighbouring opacity ideas such as Bosch smoke units belong to diesel exhaust, not this named scale.