The correct option is B Sewage treatment
The process of removing contaminants from wastewater and household sewage water is called sewage treatment. It includes physical, biological and at times chemical processes to remove pollutants. Whereas, chlorination is the process of adding chlorine to drinking water inorder to disinfect it and filtration is the process of removing impurities (suspended particles, parasites, algae etc.) thereby making it fit for drinking.