Which measurement is used to indicate exposure in an air volume?

The correct choice for indicating exposure in an air volume is the Roentgen, symbolized as "R." The Roentgen is a unit that specifically measures the amount of ionization produced in air by X-rays or gamma rays. It quantifies the exposure based on the number of ion pairs produced per unit volume of air, making it uniquely suitable for characterizing radiation exposure in that medium.

Gy (Gray) and cGy (centiGray) are units that measure absorbed dose, which accounts for the amount of energy deposited in any material, including human tissue. These measurements are not specific to air and do not convey information about the ionization process in that specific context.

Sievert is a unit used to measure biological effects of ionizing radiation, specifically dose equivalent. While it considers the impact of particular types of radiation on human tissues, it does not measure exposure in air, which is the focus of this question.