Septic tank is a 1) settling tank 2) digestion tank 3) aeration tank

C

Aeration is used in water treatment as a pretreatment in the process of removing iron and hydrogen sulfide (rotten egg smell) from water. Air is a powerful oxidizer of both iron and hydrogen sulfide. It quickly converts unfilterable ferrous iron to filterable ferric iron, and it reduces hydrogen sulfide to elemental sulphur, which is easily removed from water by a filter.

So a normal septic tank is only a settling and a digestion tank

B-  settling tank and digestion tank

Septic tank is everything that goes down any of the drains in the house (toilets, showers, sinks, laundry machines).

I would say c) (1) and (3)