In sewage treatment, flocs are:

The correct option is: the masses of bacteria associated with fungal filaments.

Explanation:

During the secondary treatment (or biological treatment) of sewage, the primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it. This allows the vigorous growth of useful aerobic microbes into flocs.

Flocs are mesh-like structures formed by the masses of bacteria that associate and entangle themselves with fungal filaments. While growing, these microbes consume the major part of the organic matter in the effluent, thereby significantly reducing the Biochemical Oxygen Demand (BOD) of the sewage water before it is discharged into natural water bodies.