The correct option is (a) Septic tank
Easiest explanation: A septic tank collects and treats wastewater at a property that is not connected to the mains sewer system. Installed underground, a septic tank makes use of natural processes to treat the sewage it stores. Usually made up of two chambers or compartments, the tank receives wastewater from an inlet pipe. Wastewater enters the first chamber and separates over time, with solids settling at the bottom, oils and greases forming a layer of scum at the top, and a layer of relatively clear water remaining in the middle.