The correct answer is option 3: swim bladder.

Fish have evolved various adaptations to help them maintain buoyancy in water. One of these adaptations is a swim bladder, which is a gas-filled organ located in the abdominal cavity of the fish. The swim bladder helps the fish control its buoyancy by adjusting the amount of gas inside it.

Option 1 - Air sinuses: While air sinuses may provide some buoyancy, they are not the main reason why most fish do not sink in water. Air sinuses are pockets of air-filled spaces found in the head region of certain fish, such as lungfishes, which allow them to extract oxygen from the air.

Option 2 - Buoyant scales on the body: While some fish have specialized scales that may contribute to their buoyancy, not all fish have these scales. Additionally, the primary reason for a fish`s buoyancy is not the presence of buoyant scales, but rather the swim bladder.

Option 4 - Swimmerets: Swimmerets are small, specialized appendages found on certain crustaceans, such as crayfish and shrimp. They are not related to fish buoyancy.

In summary, fish do not sink in water primarily due to the presence of a swim bladder, which helps