Answer:
The correct answer would be option D.
The contractile vacuole is a cell organelle which is mainly involved in the regulation of osmotic pressure.
It helps in pumping excess water out of the body.
In freshwater organisms, the inside of the cell is hypotonic as compared to water present outside the body. Consequently, the water keeps moving in the cell due to osmosis.
The contractile vacuole helps in removing this excess water out of the cell and thus, helps in regulating the osmotic pressure.