Identify the longest part of bacterial flagellum.

Filament

The correct answer is Option (4)- Filament

Motile bacteria possess thin filamentous extensions from their cell wall known as flagella. The number and arrangement of flagella in bacteria vary. The bacterial flagellum comprises three main parts: the filament, the hook, and the basal body. Among these, the filament is the longest component, extending from the cell surface outward. Composed of protein, the filament serves as the bacteria's cytoskeleton.