Which unit is part of the older system of radiation measurements?

The correct answer, Rad (RAD), is recognized as part of the older system of radiation measurements. The Rad is defined as the amount of energy absorbed per unit mass of tissue and was commonly used prior to the adoption of the more modern systems, such as the Gray and Sievert.

The Gray, which replaced the Rad, is the SI unit of absorbed radiation dose and is preferred in contemporary practices due to its precision and ease of use in scientific communication. Similarly, the Sievert is the SI unit that accounts for the biological effects of radiation, replacing the Rem.

While the Rem and Rad were widely used in earlier radiation measurement contexts, the Rad specifically is known to be part of the older system and is no longer the standard in contemporary radiation safety practices. This historical context clarifies why Rad is considered the correct answer, distinguishing it from the units of Gray and Sievert that are now standard in modern applications.