The correct answer is D) Inert gas dilutes the flammable vapor and air concentrations to keep them below the lower explosive limit. Inert gas systems on tankers function to prevent explosions in cargo tanks by reducing the concentration of oxygen in the tank atmosphere. This is achieved by injecting an inert gas, typically nitrogen or flue gas, into the cargo tanks. The inert gas dilutes the flammable vapors present in the tank, lowering the overall concentration of the flammable mixture below the lower explosive limit (LEL). This ensures that the vapor-air mixture in the tank remains non-explosive, even in the presence of an ignition source. The other options are incorrect because they do not accurately describe the primary function of an inert gas system. Option A) is incorrect as maintaining positive pressure is not the main purpose of the system. Option B) is incorrect as the inert gas does not filter out the flammable vapors, but rather dilutes them. Option C) is incorrect as the "charged mist" effect is not directly related to the inert gas system's function.