The correct answer is D) inert gas. Inert gas is used to prevent a flammable atmosphere from occurring in a cargo tank on a tank vessel. Inert gas, such as nitrogen or carbon dioxide, is injected into the cargo tank to displace the oxygen and create a non-flammable atmosphere. This prevents the formation of a flammable mixture of vapors and air, which could lead to a potentially dangerous situation. The other options are incorrect because: A) an oxidizer would increase the risk of fire, B) a vortex eliminator is used to prevent the formation of vortices during cargo transfer, and C) a combustion stabilizer is not a method for preventing a flammable atmosphere in a cargo tank.