The gas law that best explains the relationship between the pressure change and the expansion of gases inside the fish's body is A. Boyle’s Law.
Boyle’s Law states that the pressure of a gas is inversely proportional to its volume when the temperature is held constant. Therefore, as the fish is brought to the surface, the decrease in pressure causes the gases in its body (including those in the stomach) to expand, leading to the stomach protruding from its mouth.