Final answer:
Contractile vacuoles in microorganisms like protozoa function to regulate water balance by expelling excess water and wastes through exocytosis, not to be confused with nutrient storage or protein synthesis.
Step-by-step explanation:
Contractile vacuoles in microorganisms such as protozoa are specialized organelles that regulate water balance within the cell. This process is crucial for organisms living in freshwater environments where osmotic pressure could cause cells to swell and burst. The correct function of the contractile vacuole is to expel excess water and waste from the cell. They do this by a process known as exocytosis, which is the fusion of these vacuoles with the cell membrane to eject their contents into the external environment. Consequently, the answer to the student's query is:
c) Regulate water balance
This function should not be confused with that of vacuoles that store nutrients or with processes such as endocytosis or protein synthesis.