The correct option is A Marshes containing appropriate algae, fungi, and bacteria
Waste water including sewage can be treated in an integrated manner, by utilizing a mix of the artificial and the natural processes. So after the sedimentation treatment of sewage, the water still contains lots of dangerous pollutants like dissolved heavy metals to combat this the water is released into a series of marshes containing different organisms like algae, fungi, and bacteria. These microorganisms neutralize, absorb and assimilate the pollutants. Hence, as the water flows through the marshes, it gets purified naturally.