The correct statement is [A] Absolute humidity is expressed in grams per cubic meter of air. Absolute humidity represents the actual amount of water vapor present in a unit volume of air, usually measured in grams per cubic meter (g/m³). This measurement provides a direct indication of the moisture content of the air, which is essential for various applications in meteorology, engineering, and agriculture.

Options B, C, and D are incorrect. Temperature generally decreases with altitude at a rate of approximately 9.8°C per 1000 meters (not 165 meters) due to the lapse rate in the Earth’s atmosphere (B). With an increase in air temperature, the capacity of air to hold moisture actually increases, not reduces (C). An increase in temperature typically leads to a decrease in air pressure, not the opposite (D).