Dissolved nitrogen concentration in fish ponds considered favourable is:

Correct option is C

Correct Answer: (c) 0.2–0.5 ppm
Explanation:
→ Ideal dissolved nitrogen (especially un-ionized ammonia N₂) is 0.2–0.5 ppm, safe for fish health.
→ Higher concentrations can cause gill damage and reduced oxygen uptake.
Information Booster:
→ Ammonia toxicity increases with pH and temperature; regular monitoring is essential.
Additional Information:
(a) Too low but harmless.
(b) Broad range; optimal is narrower.
(d) Above 1 ppm may become toxic.