In bony fishes, the swim bladder is used for

Explanation

The question is asking about the use of the swim bladder in bony fishes. The swim bladder is an organ that helps the fish control its buoyancy in water. Option A suggests that the swim bladder is used to adjust the weight of the fish when swimming at different levels, and this option is correct. When a fish wants to change its depth in water, it adjusts the amount of gas in its swim bladder. If the fish wants to go up, it reduces the amount of gas in the swim bladder, and if it wants to go down, it increases the amount of gas in the swim bladder. This change in gas volume affects the overall density of the fish, making it easier or harder for it to float in water. Options B, C, and D are incorrect. Option B suggests that the swim bladder extracts oxygen from water, but this is not true. Fish extract oxygen from water using their gills. Option C suggests that the swim bladder equalizes fluid pressures between the fish and the surrounding water, but this is not its primary function. The swim bladder is not involved in pressure regulation in fishes. Option D suggests that the swim bladder absorbs more water when the fish is about to descend to a lower level, but this is also not true. In summary, the swim bladder in bony fishes is used to adjust the weight of the fish when swimming at different levels by changing its buoyancy. To learn more about this, please read the relevant sections of the recommended textbooks.