The water pollution in river is measured by the dissolved amount of

Correct answer: Oxygen (option 4). River water pollution is commonly assessed by measuring dissolved oxygen (DO) because the presence of organic and inorganic wastes and biological decomposition consumes dissolved O2, reducing DO and eliminating sensitive aquatic organisms such as plankton, molluscs and fish [1]. DO concentration is used as a quantitative pollution indicator: values below about 8.0 mg/L are considered contaminated and below 4.0 mg/L indicate highly polluted conditions, showing its direct link to pollution severity [2]. Dissolved oxygen is therefore a primary, widely used water-quality parameter — low DO signals eutrophication, high organic loading and reduced ecosystem health — making DO measurement the standard metric for river pollution assessment [3].

Sources

1. [1] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 5: Environmental Pollution > r. Effect$ oa aquatic eco$ystem: > p. 75
2. [2] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 5: Environmental Pollution > DO, BOD, COD > p. 76
3. [3] https://www.qmul.ac.uk/chesswatch/media/chesswatch/Dissolved-Oxygen-leaflet.pdf