The correct answer is D) Dew point temperature that the air surrounding a hygroscopic commodity will have when in moisture equilibrium with that commodity. The moisture equilibrium chart is used to determine the dew point temperature of the air surrounding a hygroscopic cargo when it is in moisture equilibrium with the cargo. This is because the dew point represents the temperature at which the air becomes saturated with moisture, and at this point, the air is in equilibrium with the moisture content of the cargo. The other options are incorrect because they do not accurately describe the primary purpose of the moisture equilibrium chart, which is to determine the dew point temperature, not the temperature, absolute moisture content, or enthalpy of the air surrounding the cargo.