The Ringelmann scale is used to measure the density of:

The Ringelmann scale (or Ringelmann chart) is a simple method used to estimate the density, or darkness, of smoke coming out of chimneys, smokestacks and vehicle exhausts. It was developed by the French professor Maximilian Ringelmann in 1888.

The scale consists of a set of cards or charts showing grids of black lines on a white background, which create different shades of grey. There are six levels, numbered 0 to 5:

An observer holds the chart at a certain distance and compares the darkness of the smoke with the shades on the chart, assigning it a Ringelmann number. Darker smoke indicates incomplete combustion and more particulate pollution, such as soot. The scale is still used by pollution control authorities in many countries to enforce limits on smoke emissions from industries, power plants and boilers, though modern instruments called opacity meters are now more common. In India, pollution control boards have used the Ringelmann scale in standards for industries and diesel vehicles.

Other measurement scales and instruments:

• Decibel (dB): intensity of sound.
• Richter scale: magnitude of earthquakes (developed by Charles Richter in 1935); the moment magnitude scale is now preferred.
• Mercalli scale: intensity of earthquakes, based on observed effects.
• Beaufort scale: wind speed, from 0 (calm) to 12 (hurricane).
• Mohs scale: hardness of minerals, from 1 (talc) to 10 (diamond).
• pH scale: acidity or alkalinity.
• Saffir-Simpson scale: intensity of hurricanes.
• Fujita scale: intensity of tornadoes.
• Dobson unit: thickness of the ozone layer.
• Air Quality Index (AQI): India's National AQI was launched in 2014.

Why not the other options?

• ✗Sound is measured in decibels.
• ✗Earthquakes are measured on the Richter or moment magnitude scale.
• ✗Water hardness is measured in parts per million (ppm) of calcium carbonate.