The correct option is D Flagella
The most common mechanism used by bacteria to swim through liquid media is the flagellum. The bacterial flagellum consists of 3 major domains: an ion driven motor, which can provide a torque in either direction; the hook, a universal joint, which transmits motor torque even if it is curved; and the filament, a very long structure, which acts as a propeller, and behaves differently depending on, which way the motor turns.