The correct option is C Osmoregulation
Contractile vacuoles are temporary excretory structures of protists like protozoans (Amoeba). The contractile vacuole pumps excess water out of the cell. In freshwater environments the concentration of solutes inside the cell is higher than outside the cell (i.e., the environment is hypotonic). Under these conditions water flows from the environment into the cell by osmosis. The contractile vacuole serves as a protective mechanism that prevents the cell from absorbing too much water and possibly exploding.