The correct option is C Phages
River water begins to putrefy when lack of oxygen promotes the growth of anaerobic bacteria, which produce the tell-tale smell of stale water. While the organic wastes, that are poured into Ganga at different locations, complicate the ecosystem by the disintegration of the biodegradable material leading to an upset in the oxygen balance, also have the capacity to fertilize the water which in turn enriches the aquatic biomass. The presence of bacteriophages which are widely distributed in locations populated by bacteria are found in the river water and this helps the Ganges from being putrefied.