How does the contractile vacuole in a single-celled organism function to maintain homeostasis?
It pumps water into the cell.
It pumps water out of the cell.
It pumps solutes into the cell.
It pumps solutes out of the cell.

It pumps water out of the cell; as water enters the organism's cells via osmosis (diffusion of water), the contractile vacuole acts very much like a pump to allow water to exit the organism; without it the organism would die. Also, homeostasis is maintained in this manner; all it means is a condition of stability.