Which of the following has least permissible limit in drinking water ?

Options

A. Chlorides
B. Dissolved oxygen
C. Zinc
D. Lead.

Answer

To determine which substance has the least permissible limit in drinking water from the options provided, we will analyze the permissible limits of each substance mentioned:

1. **Chloride**: The permissible limit is 250 mg per litre.
2. **Dissolved Oxygen**: The permissible limit is between 6.5 mg per litre and 8 mg per litre. For our comparison, we will take the lower limit, which is 6.5 mg per litre.
3. **Zinc**: The permissible limit is 5 mg per litre.
4. **Lead**: The permissible limit is 15 micrograms per litre (which is equivalent to 0.015 mg per litre).

Now, we will convert all the permissible limits to the same unit for a fair comparison:

- Chloride: 250 mg/L
- Dissolved Oxygen: 6.5 mg/L
- Zinc: 5 mg/L
- Lead: 0.015 mg/L

Now, we can compare these values:

- Chloride: 250 mg/L
- Dissolved Oxygen: 6.5 mg/L
- Zinc: 5 mg/L
- Lead: 0.015 mg/L

From the comparison, we can see that Lead has the least permissible limit in drinking water.

**Conclusion**: The substance with the least permissible limit in drinking water is **Lead**.