The correct answer is D) Inert gas dilutes the flammable vapor and air concentrations to keep them below the lower explosive limit. The purpose of an inert gas system on a tanker is to maintain an inert atmosphere in the cargo tanks to prevent the formation of an explosive mixture of flammable vapors and air. By introducing an inert gas, such as nitrogen or flue gas, the system dilutes the concentration of flammable vapors in the tank to below the lower explosive limit (LEL). This ensures that even if an ignition source is present, an explosion cannot occur because the mixture is not combustible. The other answer choices do not accurately describe how an inert gas system functions to prevent explosions in cargo tanks.