The WHO standard of drinking water for nitrate is

Correct option is B

The correct answer is $$(B) 10 mg NO3−−N/L10 \text{ mg NO}_3^--N/L10 mg NO3−​−N/L$$​​

Explanation:
• The World Health Organization (WHO) and various environmental agencies set standards for nitrate levels in drinking water to protect public health.
• There are two ways to express this limit: 1. As Nitrate ($NO_3$): The limit is $45-50 mg/L$. 2. As Nitrate-Nitrogen ($NO_3-N$): The limit is $10 mg/L$.
• $10 mg/L$ of Nitrate-Nitrogen is approximately equivalent to $44.3 mg/L$ of Nitrate.
• The options provided use the $NO_3-N$ notation, therefore $10 mg/L$ is the correct threshold.

Information Booster:
• This limit is primarily set to prevent Methemoglobinemia, or "Blue Baby Syndrome." Infants have a less acidic stomach which allows bacteria to convert nitrate to nitrite, which then binds to hemoglobin and reduces the blood's oxygen-carrying capacity.
• High nitrate in water is often an indicator of agricultural runoff or sewage contamination.

Additional Knowledge:
• In India, the Bureau of Indian Standards (BIS) also follows the $45 mg/L$ (as $NO_3$) limit for drinking water.
• Treatment for high nitrate include reverse osmosis, ion exchange, and electrodialysis; boiling water does not remove nitrate (it actually concentrates it).