Ans.
The process that involves purification of raw sewage is known as sewage treatment. It removes contaminants from wastewater, mainly from industrial wastewater and household sewage (wastewater produced by people) and produces treated water, which is safe enough to release in environment. The by-product of sewage treatment is further treated to make suitable to release or dispose to land.
Thus, the correct answer is 'option). sewage treatment.'