Given below are two statements:
Statement-I: If BOD is 4 ppm and dissolved oxygen is 8 ppm, then it is a good quality water.
Statement-II: If the concentration of zinc and nitrate salts are 5 ppm each, then it can be a good quality water.
In the light of the above statements, choose the most appropriate answer from the options below:

We need to evaluate two statements about water quality parameters.

Evaluating Statement I: "If BOD is 4 ppm and dissolved oxygen is 8 ppm, then it is a good quality water."

BOD (Biochemical Oxygen Demand) measures the amount of dissolved oxygen consumed by microorganisms to decompose the organic matter present in water. A lower BOD indicates less organic pollution.

- Clean water typically has a BOD of less than 5 ppm.

- Water is considered polluted when BOD exceeds 17 ppm.

Dissolved oxygen (DO) indicates the amount of oxygen available in water for aquatic organisms. Higher DO values indicate better water quality.

- Good quality water has DO around 6-8 ppm or higher.

Given BOD = 4 ppm (which is less than 5 ppm, so acceptable) and DO = 8 ppm (which is in the good range), this water qualifies as good quality water. Statement I is correct.

Evaluating Statement II: "If the concentration of zinc and nitrate salts are 5 ppm each, then it can be good quality water."

The permissible limits for various ions in drinking water as per standard guidelines are:

- Zinc: The permissible limit is approximately 5 ppm. A concentration of 5 ppm of zinc is within the acceptable range.

- Nitrate (NO$$_3^-$$): The permissible limit for nitrate in drinking water is approximately 45-50 ppm. A concentration of 5 ppm of nitrate is well below this limit.

Since both zinc (5 ppm) and nitrate (5 ppm) are within their respective permissible limits, the water can be considered good quality. Statement II is correct.

Both statements are correct. The correct answer is Option 4: Both the statements I and II are correct.