Find the incorrect statement

The correct option is: "dissolved oxygen concentration below 5 ppm is ideal for the growth of fish"

Let us analyze each statement step-by-step to understand why this statement is incorrect and therefore the correct option for this question:

1. Analysis of Option 1: "BOD value of clean water is less than 5 ppm"
Biochemical Oxygen Demand (BOD) is a measure of the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given water sample at a certain temperature over a specific time period. Clean water has low organic pollution, resulting in a very low BOD value, typically less than 5 ppm (parts per million). Highly polluted water, on the other hand, has a BOD value of 17 ppm or more. Thus, this statement is correct.

2. Analysis of Option 2: "Drinking water pH should be between 5.5-9.5"
The standard pH range guidelines for drinking water prescribed by many environmental protection agencies (including the Bureau of Indian Standards and WHO guidelines) generally specify an acceptable pH range between 6.5 and 8.5, or a broader permissible limit of 5.5 to 9.5 in the absence of an alternate source. Therefore, this statement is considered chemically and environmentally acceptable and correct.

3. Analysis of Option 3: "carbon, sulphur and nitrogen oxides are the most widespread air pollutants"
Oxides of carbon (such as CO and CO₂), oxides of sulfur (such as SO₂ and SO₃), and oxides of nitrogen (such as NO and NO₂) are released in massive quantities from the combustion of fossil fuels in industries, power plants, and vehicles. They represent the most widespread gaseous air pollutants globally. Thus, this statement is correct.

4. Analysis of Option 4: "dissolved oxygen concentration below 5 ppm is ideal for the growth of fish"
Dissolved Oxygen (DO) is vital for the survival of aquatic organisms. Clean, healthy water bodies typically maintain a dissolved oxygen concentration of around 8 ppm. If the concentration of dissolved oxygen falls below 6 ppm, it becomes stressful for fish. A dissolved oxygen concentration below 5 ppm is highly detrimental, inhibits growth, and can cause suffocation and death in many fish species. Therefore, a DO level below 5 ppm is definitely not ideal for the growth of fish, making this statement incorrect.