The process of self-purification process of polluted waters can be noted by the

Correct option is D

Explanation of the Correct Option (d): Physical, chemical, and biological changes
· Self-purification of water is a natural process by which polluted water bodies restore their quality.
· It involves three types of changes:
Physical changes
· Sedimentation of suspended solids
· Dilution and aeration
Chemical changes
· Oxidation of organic matter
· Reduction in BOD and COD
· Changes in dissolved oxygen
Biological changes
· Decomposition of organic matter by microorganisms
· Algal growth and microbial activity
· Natural food chain regulation
· Since all three processes occur simultaneously, option (d) is correct.
Explanation of Other Options
Option (a): Physical changes only
· Incorrect:
· Does not account for biochemical processes.
Option (b): Chemical changes only
· Incorrect:
· Biological activity plays a major role.
Option (c): Biological changes only
· Incorrect:
· Physical and chemical processes are equally important.
Key Points
· Self-purification occurs in rivers, lakes, and streams
· Depends on:
· Flow velocity
· Temperature
· Oxygen availability
· Microbial population