Answer:
4. Gas vacuoles
Explanation:
The other structures in options may also be present in other bacteria but gas vacuoles are present specifically in aquatic bacteria. Gas vacuoles are composed of clusters of gas vesicles and found inside the bacterial. Gas vacuoles interior is hydrophobic to avoid water.
Gas vacuoles membrane is permeable to certain gases such as carbon dioxide and they control the buoyancy (floating) of bacterial cell in water by becoming inflated and deflated.
Example; Cyanobacteria
