The correct answer is D) Cargo hold moisture content is reduced by supplying relatively dry air from the outside and exhausting relatively humid air to the outside. This is correct because dehumidification of cargo holds involves removing moisture from the air inside the hold. By supplying dry air from the outside and exhausting the relatively humid air from the hold, the overall moisture content within the cargo hold is reduced. This helps prevent cargo damage and preserve the quality of goods being transported. The other options are incorrect because they describe the opposite process - either increasing the moisture content or using humid air instead of dry air for dehumidification, which would not effectively remove moisture from the cargo hold.