Concept:The contractile vacuole is a specialised organelle in protozoans that pumps out excess water to maintain osmotic balance.
Explanation:Protozoans like Amoeba live in freshwater.
The surrounding water enters their body continuously by osmosis.
If this water were not removed, the cell would swell and burst.
The contractile vacuole collects the excess water from the cytoplasm.
When full, it moves to the body surface and contracts.
This contraction forces the water out of the protozoan's body.
Thus, the contractile vacuole serves as the organelle for eliminating water.
This process of expelling water is called osmoregulation.
It is essential for survival in hypotonic freshwater environments.
Other options are incorrect: the plasma membrane is a boundary, the nucleus controls metabolism, and a cell wall is absent in protozoans.
Answer:B. contractile vacuole