The correct option is C Contractile vacuole
Osmoregulation is the process which takes place in unicellular protist. It regulates the osmotic pressure inside the cell. Vacuoles are the organs which are known for osmoregulation. It maintains the level of salt and water inside a cell. If the concentration of the salt is more inside the body the water is absorbed while if the extra water is released if salt concentration is less than the surrounding environment. Thus, the correct answer is option C.