The correct answer is B) dilute tank atmospheres to keep gas concentrations below the lower explosive limit. The inert gas system on a tanker is used to reduce the oxygen content in cargo tanks and other enclosed spaces to a level that prevents the formation of an explosive atmosphere. By displacing the oxygen with an inert gas like nitrogen or flue gas, the system keeps the gas concentration below the lower explosive limit, which is the minimum concentration of a flammable gas or vapor in air needed for ignition to occur. This helps prevent fires and explosions in the cargo tanks. The other answer choices are incorrect because they do not accurately describe the primary purpose of the inert gas system on a tanker. The system is not used to blow out cargo lines, prevent fires in the pump room, or prevent the generation of flammable gases in the tanks.