The correct option is C Microorganisms
Sewage pollution can be prevented by treating sewage before passing into water course, in sewage or effluent treatment plants (ETP). The sewage is treated in ETP in three stages. Stage one is Primary Treatment which comprises of physical processes like sedimentation. Second Stage is aerobic and anaerobic biological degradation by microorganisms. Last or tertiary stage is physicochemical degradation for removing turbidity and dissolved substances.