Answer: The correct answer for the fill in the blank is- Vacuole.
Vacuole or contractile vacuole is a specialised subcellular organelle that is responsible for osmoregulation ( balance of water and electrolyte concentration).
For instance, in freshwater environment ( which is hypotonic), it allows the water to accumulate inside the cell.
It is predominantly found in protists ( such as Amoeba and Paramecium) and unicellular algae. It prevents the organism from rupturing or lysing as it controls the input and output of water.
