The correct answer is C) lower the oxygen levels inside cargo tanks, making explosion nearly impossible. Inert gas systems on tankers are designed to reduce the oxygen content inside cargo tanks, which in turn makes it nearly impossible for explosions to occur. By displacing oxygen with an inert gas like nitrogen, the system creates a non-flammable atmosphere inside the tanks, effectively eliminating the risk of fire or explosion. The other answer choices are incorrect because they do not accurately describe the primary purpose of an inert gas system on a tanker. Option A is focused on tank cleaning, while option B is about venting toxic fumes, and option D is about increasing cargo discharge rate - none of which are the main function of an inert gas system.