The correct answer is option 4. DDT is not effective in removing water pollution, therefore it is not used frequently.

Let`s delve into each option to understand why option 4 is incorrect.

Option 1 states that by increasing the plant population, we can reduce the concentration of carbon dioxide. This statement is correct because plants absorb carbon dioxide during photosynthesis, thus reducing its concentration in the atmosphere.

Option 2 claims that the soot particles produced by industries are collected by leaf surfaces of plants. This statement is also accurate as the leaves of plants can act as surfaces for the deposition of airborne particles, including soot.

Option 3 suggests that some plants can grow well in polluted water and remove toxic compounds and heavy metals. This statement is true because certain plant species have the ability to absorb and accumulate pollutants, which helps in mitigating water pollution.

Option 4, as stated earlier, is not correct. DDT is a pesticide that is effective in controlling insects, particularly mosquitoes. It is not primarily used for removing water pollution.

Therefore, the correct answer is option 4, and it should be noted that DDT is not effective in removing water pollution.