The correct answer is C) dilute tank atmospheres to keep gas concentrations below the lower explosive limit. Inert gas systems on tankers are used to displace flammable vapor inside cargo tanks and keep the atmosphere below the lower explosive limit (LEL). This prevents the formation of an explosive gas-air mixture that could ignite and cause a fire or explosion. Option C correctly describes the primary purpose of an inert gas system on a tanker. The other options are incorrect. Option A refers to preventing the generation of flammable gas, which is not the main function of the inert gas system. Option B is about blowing out cargo lines, which is a separate system. Option D is about preventing fires in the pump room, which is not the main purpose of the inert gas system on the cargo tanks.