**Core Concept**
Radiation dose measurement is a critical aspect of radiation oncology, nuclear medicine, and diagnostic imaging. The unit of measurement for radiation dose is essential to ensure accurate delivery and minimize harm to patients. The correct unit of measurement is based on the International System of Units (SI) and is used worldwide.

**Why the Correct Answer is Right**
Radiation dose is measured in Grays (Gy), which is the SI unit of absorbed radiation dose. One Gray is equal to the absorption of one joule of radiation energy per kilogram of matter. This unit is derived from the Gyroscopic effect, where the radiation energy is absorbed by the mass. The Gy unit is widely used in radiation therapy, nuclear medicine, and diagnostic imaging to ensure accurate dose delivery. In contrast, other options like sieverts (Sv) measure the biological effect of radiation.

**Why Each Wrong Option is Incorrect**
**Option A:** Gray is incorrect because it is the correct unit of measurement for absorbed radiation dose, but not the one that measures the biological effect.
**Option B:** Röntgen is incorrect because it is an older unit of measurement that has been largely replaced by the Gray and Sievert.
**Option C:** Rem is incorrect because it is a unit of measurement for the biological effect of radiation, but not the absorbed radiation dose.

**Clinical Pearl / High-Yield Fact**
Remember that the Gray (Gy) measures the absorbed radiation dose, while the Sievert (Sv) measures the biological effect of radiation. When ordering radiation therapy or nuclear medicine tests, ensure that the correct unit of measurement is used to avoid errors.

**Correct Answer:** C. Rem