Which unit refers to the level of ionization in the air?

The correct answer, which refers to the level of ionization in the air, is the roentgen. The roentgen is a unit specifically used to measure exposure to ionizing radiation in the air, representing the amount of ionization produced in air by X-rays or gamma rays. It quantifies the radiation exposure in terms of the ion pairs produced in a specified volume of air.

In medical imaging and radiation safety, understanding the level of ionization in the air is crucial because it helps assess the potential biological effects of radiation on humans. The roentgen is primarily focused on measuring the exposure and is relevant in contexts like radiology and radiation protection.

Other units listed, such as rad, sievert, and gray, refer to different aspects of radiation: the rad and gray are used to measure absorbed dose (the energy deposited in any material), while the sievert is used to measure biological effects of radiation, taking into account the type of radiation and its impact on human tissue. Therefore, while important, these units do not specifically address the ionization level in air.