Correct Option (d)
Dissolved Oxygen (DO) is a crucial parameter for assessing water quality in rivers and other aquatic ecosystems. It represents the amount of oxygen gas dissolved in water, which is essential for the survival of aerobic aquatic organisms such as fish, invertebrates, and microorganisms. A higher DO level generally indicates good water quality, supporting diverse aquatic life. Conversely, a low DO level is indicative of water pollution, often resulting from the decomposition of organic matter by aerobic bacteria, which consumes oxygen, or the presence of other oxygen-depleting substances. While Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are also key indicators of organic pollution, DO directly measures the available oxygen.
Incorrect Options
Chlorine (a): Chlorine is primarily used as a disinfectant in water treatment to kill pathogens. While its presence can indicate treatment or contamination, it is not a direct measure of the overall pollution level in terms of the dissolved amount that supports aquatic life or indicates organic load.
Ozone (b): Ozone is a powerful oxidizing agent and can be used for water purification. However, it is not a standard or commonly measured parameter for assessing general river pollution levels in the context of dissolved amounts.
Nitrogen (c): Nitrogen, particularly in forms like nitrates and ammonia, is a significant nutrient pollutant that can lead to eutrophication in aquatic ecosystems. While its concentration indicates pollution, the question specifically asks about the 'dissolved amount' that measures pollution in a broad sense, referring to the availability of oxygen for aquatic life and decomposition processes, which is distinct from nitrogen's role as a nutrient pollutant.