Sediment left after secondary treatment of sewage/waste is known as:

The correct answer is Option (3) → Activated Sludge

Option (3) Activated Sludge — Correct. In secondary treatment, bacteria grow into flocs in the aeration tank. The effluent is passed into a settling tank where these bacterial flocs settle as sediment, called activated sludge; a small part of it is pumped back into the aeration tank as inoculum.

NCERT Extract:
Microbes in Sewage Treatment – “… Once the BOD of sewage or waste water is reduced significantly, the effluent is then passed into a settling tank where the bacterial ‘flocs’ are allowed to sediment. This sediment is called activated sludge. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum.”