The correct answer is A) prevent outside air from entering the tank. Inert gas systems aboard tank vessels are used to maintain a non-flammable atmosphere inside the cargo tanks. By displacing the oxygen in the tank with an inert gas like nitrogen or carbon dioxide, the system prevents the buildup of flammable vapors that could lead to an explosion. This is a critical safety measure to protect the vessel and crew. The other answer choices are incorrect because: B) double hull regulations do not require inert gas systems, C) inert gas systems are not used to increase cargo discharge pressure, and D) the purpose is to create a non-flammable atmosphere, not to sustain life inside the tank.