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

The correct option is B.

In single celled animals, contractile vacuole function in osmoregulation by regulating the quantity of water that is found in the cell, thereby regulating the osmotic pressure of the cell. Contractile vacuoles are usually found in freshwater protists.
The cells of unicellular organisms operate by osmosis, therefore, the water and the solute contents of the cell have to be balanced. Thus, if there is too much water in the cell, the contractile vacuole functions by pumping the water out of the cell. This helps to protect the cell from bursting, because if the quantity of water inside the cell is too much, the cell will swell up and burst.