Final answer:
A flagellum is akin to a propeller for a bacterium, as both provide locomotion – the flagellum through whipping motions in a cellular context, and a propeller through rotations in water for a boat.
Step-by-step explanation:
A flagellum is to a bacterium as a propeller is to a boat. Bacteria depend on flagella to perform the vital function of movement similar to how a boat uses a propeller to move through water. Flagella are long, whip-like structures that extend from the plasma membrane of various cells, including some bacteria, and they operate to propel the cell by rotating, much like a propeller spins to propel a boat forward.