The correct answer is option 2: Hydrostatic organ.

The air bladder in fish is a gas-filled organ that helps them control their buoyancy in water. It acts as a hydrostatic organ by regulating the fish`s overall density, allowing it to adjust its position in the water column. The air bladder can be inflated or deflated with gases, usually oxygen or nitrogen, allowing the fish to stay at different depths in the water. By decreasing the volume of the air bladder, the fish becomes denser and sinks, while increasing the volume makes it less dense and enables it to rise.

It is important to note that while the air bladder in some fish species can also serve as an accessory respiratory organ, enabling them to extract oxygen from the air, this is not the primary function of the air bladder in most fish.