**Core Concept**
Bacteria lack a true nervous system and do not possess organs in the classical sense. However, they do have specialized structures that facilitate movement. This question aims to test the student's understanding of the fundamental characteristics of bacterial physiology.

**Why the Correct Answer is Right**
The correct answer is the flagellum, which is a whip-like structure that protrudes from the bacterial cell wall. The flagellum is composed of a helical filament made of flagellin proteins, surrounded by a hook and a basal body. The flagellum rotates to generate torque, allowing the bacterium to move through its environment. This movement is essential for bacterial locomotion, chemotaxis, and the colonization of new environments.

**Why Each Wrong Option is Incorrect**
**Option A:** While the bacterial cell wall provides structural support and maintains the cell's shape, it is not responsible for locomotion.
**Option B:** The cell membrane is a vital component of the bacterial cell, but it does not play a direct role in movement.
**Option C:** The periplasmic space is a region between the inner and outer membranes of the bacterial cell, but it is not an organ of locomotion.

**Clinical Pearl / High-Yield Fact**
Bacterial flagella are often used as a target for antibiotics that inhibit bacterial motility, such as quinolones and fluoroquinolones.

**Correct Answer: C. Flagellum.**