The correct answer is C) inert gas and dry air with a minimum dew point of -40°F. The inert gas plant on an LNG vessel is designed to produce a mixture of inert gas and dry air with a very low dew point. This is necessary to maintain an inert atmosphere in the cargo tanks and other spaces to prevent the buildup of flammable gas mixtures. The dew point requirement of -40°F ensures the gas mixture is sufficiently dry to prevent condensation and the associated risks. The other options are incorrect as they do not accurately describe the full purpose and specifications of the inert gas plant on an LNG vessel.