The correct answer is B) tank atmosphere with low oxygen content. The primary advantage of using an inert gas system on a tank vessel is that it provides a tank atmosphere with low oxygen content. This is important for safety, as it reduces the risk of explosions or fires within the cargo tanks. By displacing the oxygen with an inert gas, such as nitrogen or flue gas, the tank atmosphere becomes less combustible, which helps prevent accidental ignition of flammable cargo vapors. The other options are not the primary advantages of an inert gas system. Option A, better fuel economy, is not directly related to the purpose of an inert gas system. Option C, for faster loading, is also not a primary advantage, as the inert gas system is more focused on maintaining a safe tank atmosphere during cargo operations.