Final answer:
The gas-filled sac used by bony fishes to control buoyancy is called the swim bladder, which allows them to maintain their position in water without continuous swimming.
Step-by-step explanation:
A gas-filled sac that is used to control buoyancy in fish is known as a swim bladder. This organ is derived as a pouch from the gut, and helps bony fishes maintain their position in the water column without having to expend energy swimming. The swim bladder works by varying the amount of gas (mostly oxygen) inside it, which changes the density of the fish relative to the water, allowing it to move up or down. Unlike sharks, which do not have this organ, bony fishes, including both ray-finned fishes (Actinopterygii) and lobe-finned fishes (Sarcopterygii), possess a swim bladder for buoyancy control. The correct answer to the student's question is A) Swim bladder.