## Core Concept
The core concept being tested here is the measurement of radiation exposure. In medical physics, particularly in the context of radiation safety and dosimetry, it's crucial to understand the units used to quantify radiation exposure.

## Why the Correct Answer is Right
The correct answer, **Sievert (Sv)**, is a unit used to express the biological effect of radiation exposure, specifically the equivalent dose. It takes into account the relative biological effectiveness (RBE) of different types of radiation. The Sievert is defined as the product of the absorbed dose in Grays (Gy) and a quality factor (Q) that depends on the type of radiation. This makes Sv a more comprehensive measure for assessing the risk of radiation exposure to humans.

## Why Each Wrong Option is Incorrect
- **Option A:** The Rad is an older unit that measures absorbed radiation dose, not exposure. It does not account for the biological effect of the radiation.
- **Option B:** The Roentgen (R) is a unit of exposure for X-rays and gamma rays in air, but it does not directly measure the biological effect or the dose absorbed by tissues.
- **Option D:** The Gray (Gy) measures the absorbed dose of radiation but does not account for the biological effectiveness of different types of radiation.

## Clinical Pearl / High-Yield Fact
A key point to remember is that for **X-rays and gamma rays**, the Sievert (Sv) and Gray (Gy) are numerically equal because the quality factor (Q) for these types of radiation is 1. However, Sv is used for protection purposes and relates to the biological effect, while Gy is used for absorbed dose.

## Correct Answer: C. Sievert.