Identify the longest part of the bacterial flagellum.

Correct option is D

The longest part of the bacterial flagellum is the filament. The filament is a hollow, rigid cylinder, composed of a single type of protein, flagellin. It is the propeller of the flagellum, rotating to move the bacterium forward.
A bacterial flagellum is composed of three parts: the filament, the hook, and the basal body. The filament is the longest part of the flagellum and extends from the cell surface to the outside. It is made up of a protein called flagellin.
The hook is a short, curved structure that connects the filament to the basal body.
The basal body is a complex structure that is embedded in the cell membrane. It is responsible for the rotation of the flagellum.