What is the SI measurement for radiation exposure in the air?

The correct measurement for radiation exposure in the air is Gray-a (Gy-a), which is a subunit of the Gray unit specifically used for air kerma. The Gray is a measurement of the absorbed dose of ionizing radiation, which indicates the amount of radiation energy absorbed by a material, in this case, air.

In the context of radiation exposure, including X-rays, the SI unit outlines that "Gy" represents the energy absorbed per kilogram of matter. When we specify "a" (air), it indicates that this absorbed dose is particularly relevant to exposure in air, which is crucial for safety and dosimetry calculations in radiological practices.

The other choices refer to different units or concepts related to radiation. For instance, Rad-a pertains to a non-SI unit of absorbed dose, and Roentgen is an outdated measure of exposure to X-rays and gamma rays specifically in air but not strictly an SI unit. Gray-t refers to the Gray for tissue dose, which differs in application from the air kerma or exposure context specified in the question. Thus, Gray-a is the most accurate answer for measuring radiation exposure in air.