**Core Concept**
The question is testing the understanding of radiation exposure units, which is a fundamental concept in radiation safety and protection in medicine. Radiation exposure is measured in terms of the ionizing effect of radiation on living tissues, and it is essential to understand the different units used to express this effect.

**Why the Correct Answer is Right**
The correct unit of radiation exposure is the **Sievert (Sv)**. The Sievert takes into account the biological effect of radiation on the human body, which is a critical aspect of radiation protection. The Sievert is defined as the dose of radiation that increases the risk of cancer by 1% in 100 people over a lifetime. The Sievert is a more sensitive measure than the Gray (Gy), which only measures the absorbed dose of radiation.

**Why Each Wrong Option is Incorrect**
* **Option A:** The **Gray (Gy)** is a unit of absorbed radiation dose, but it does not take into account the biological effect of radiation. Therefore, it is not the correct unit of radiation exposure.
* **Option B:** The **Rem (R)** is an older unit of radiation exposure, but it is no longer used in modern medical practice. It is equivalent to 1/100 of the Sievert and does not accurately reflect the biological effect of radiation.
* **Option C:** The **Roentgen (R)** is a unit of exposure to ionizing radiation, but it is not a measure of the biological effect of radiation. It is used primarily in the context of X-ray exposure.

**Clinical Pearl / High-Yield Fact**
Remember that the Sievert is the correct unit of radiation exposure because it takes into account the biological effect of radiation on the human body. This is a critical aspect of radiation protection in medicine, and it is essential to understand the difference between absorbed dose and biological effect.

**Correct Answer:** . Sievert (Sv).