Assertion: During secondary treatment of sewage, the primary effluent is passed into large settling tanks and is constantly agitated mechanically and air is pumped into it.
Reason: This allows the vigorous growth of the useful anaerobic microbes.

The correct option is D Both the assertion and reason are false
Sewage is the municipal waste water that is generated in cities and towns. The sewage is treated in the sewage treatment plant in two stages: primary treatment and secondary treatment.

During secondary treatment of sewage, primary effluent collected after the primary treatment is passed into the large aeration tanks. The effluent is agitated mechanically and air is pumped into it. This causes enormous growth of the aerobic microbes. These microbes form mesh-like structures called flocs. Flocs are masses of fungal filaments and bacteria forming a mesh-like structure. They digest the organic matter present in the effluent. This eventually decreases the BOD.
Once the BOD is reduced, it is passed into large settling tanks, where the flocs sediment. This sediment is called activated sludge which is digested by anaerobic bacteria in anaerobic sludge digesters. The effluent is released into the natural water bodies like rivers and streams.
Hence, both the assertion and reason are false.