Looking into a microscope, you notice cells swimming, propelled by a long tail. what cell structure must these cells have in order to be mobile: ribosomes cytoplasm flagella peroxisomes smooth er

The correct option is FLAGELLA.
Flagella is designed primarily for locomotion, although it used additionally as a sensory organelle in some cells been sensitive to chemicals in and out of the cells. Flagella allow cells to move from one point to another. The flagella acts as a propeller which the organism uses to move itself from one location to another while rotating either in a clockwise or an anticlockwise direction.