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.

"Option B- It pumps water out of the cell".

Explanation:

Vacuoles in the cell organism are responsible to maintain the amount of water in the cell body, to get stable in the environment vacuoles either  remove or absorb the water through the mechanism of osmosis or osmoregulation.

Homeostasis is the phenomenon of maintaining balance of physical and chemical conditions of organism according to the environmental conditions to survive in it.

In single celled organisms, contractile vacuole pumps out the water from the cell through osmoregulation and excess of water is pumped out by the contractile vacuole.      

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the answer is lactic acid from anaerobic respiration. the acid in lactic acid is what creates sore and burning muscles. if you think about how acid burns that will give you a clue for questions like this.