The correct answer is Flagella. Flagella are long, whip-like structures that extend from the bacterial cell surface and enable active movement by rotation. This motility allows bacteria to navigate toward favorable environments or away from harmful stimuli, a process known as chemotaxis. Unlike pili, capsule, or cell wall, flagella are specifically designed for locomotion.

Reference: Jawetz, Melnick & Adelberg's Medical Microbiology, 27th Edition, Chapter 3, p. 45