The part labelled II in the diagram is the contractile vacuole. A contractile vacuole is an organelle found in certain unicellular organisms that helps to regulate water content within the cell. It works by actively pumping excess water out of the cell, which helps to prevent the cell from bursting due to osmotic pressure. The contractile vacuole is easily identified in the diagram due to its spherical shape and proximity to the cell membrane.
In the diagram, the part labelled II is the