Concept:The part labelled a is the operculum, a protective gill cover in bony fish.
Explanation:In fish such as tilapia, the operculum is the flap that covers and protects the gills.
It plays a direct role in breathing underwater.
When the operculum opens and closes, it helps water flow steadily over the gills.
Oxygen dissolves in this water and is taken up by the gill filaments.
At the same time, carbon dioxide from the fish’s blood is released into the water.
Therefore, the operculum is mainly concerned with gaseous exchange between the fish and its aquatic environment.
It is not used for detecting pressure, maintaining buoyancy, or swimming movement.
Answer:C. Gaseous exchange
