What is the name given to a small vesicle in the cytoplasm of many freshwater protists that expels excess water?

The correct choice refers to a structure known as the contractile vacuole. This specialized organelle plays a crucial role in osmoregulation, which is the process by which cells maintain the balance of water and solutes. In freshwater protists, which often exist in a hypotonic environment (where the concentration of solutes outside the cell is lower than inside), excess water continually enters the cell through osmosis.

The contractile vacuole forms to collect this excess water and, when fully expanded, it contracts to expel the water out of the cell, thus preventing the cell from bursting due to osmotic pressure. This mechanism is vital for the survival of freshwater protists, allowing them to thrive in their aquatic habitats by maintaining internal stability amidst fluctuating external conditions.

The other terms listed, such as cell vacuole, water vacuole, and osmotic vacuole, do not specifically refer to this function or structure in the context of freshwater protists and therefore do not accurately describe the vesicle responsible for expelling excess water.