In water quality assessment, dissolved oxygen (DO) is a critical indicator. A water sample shows DO of 3.2 mg/L. According to standard classification, this level indicates the water is:

• A Excellent, suitable for drinking without treatment
• B Suitable for propagation of fisheries and aquatic life
• C Acceptable only for industrial cooling purposes
• D Polluted, unable to support fish survival ✓

Explanation

Dissolved oxygen below 4 mg/L is generally considered inadequate for survival of fish and most aquatic organisms. Standard texts classify DO less than 4 mg/L as polluted water that cannot support fisheries. The 4 mg/L threshold is a widely accepted minimum for warm-water fish survival. Option B is wrong because supporting fisheries requires DO above 5 mg/L.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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