**Core Concept**
The question is testing the understanding of radiation exposure measurement units. In medical physics, radiation exposure is a critical parameter measured to assess the biological effects of ionizing radiation. The unit of measurement for radiation absorbed dose is a fundamental concept in radiation protection and therapy.

**Why the Correct Answer is Right**
The correct unit of radiation absorbed dose is the Gray (Gy). The Gray is defined as the absorption of one joule of radiation energy per kilogram of matter. This unit is used to quantify the amount of radiation energy deposited in a material or biological tissue. The Gray is a fundamental unit in radiation protection and is used to calculate the absorbed dose in various medical applications, including radiation therapy and diagnostic imaging.

**Why Each Wrong Option is Incorrect**
**Option A:** Rad - Although the Rad is an older unit of radiation absorbed dose, it has been largely replaced by the Gray in international use. The Rad is defined as 100 ergs of energy deposited per gram of material, which is equivalent to 10^-2 Gy.

**Option B:** Sievert - The Sievert (Sv) is a unit of radiation dose equivalent, which takes into account the biological effects of radiation. While related to absorbed dose, the Sievert is not a unit of absorbed dose itself.

**Option C:** Rem - The Rem is an older unit of radiation dose equivalent, similar to the Sievert. It is not a unit of absorbed dose and has been largely replaced by the Sievert.

**Clinical Pearl / High-Yield Fact**
In radiation protection, the absorbed dose is a critical parameter in assessing the biological effects of ionizing radiation. The Gray is a fundamental unit of measurement, and understanding its definition and application is essential in medical physics and radiation safety.

**Correct Answer: C. Gy**