Final answer:
Flagella move in a counterclockwise direction for smooth forward motion or swimming in bacteria, known as a run. Clockwise rotation causes random movement called tumbling, which is not suitable for smooth swimming.
Step-by-step explanation:
Bacteria achieve smooth forward motion, or swimming when their flagella rotate in a counterclockwise direction. This counterclockwise rotation causes the flagella to bundle together, propelling the bacterium forward in a movement referred to as a run. In contrast, when flagella rotates clockwise, this leads to a tumble, which is a random and circuitous movement that does not result in meaningful forward displacement but serves to reorient the bacterium.
Therefore, to have smooth forward motion/swimming, flagella must move in a counterclockwise direction (Option b). This mode of motion is especially prominent in bacteria responding to environmental signals, such as chemical gradients in a process known as chemotaxis, where cells increase the length of their runs to move toward an attractant and decrease the length of tumbles.