**Core Concept**
The question is testing the understanding of the unit of measurement for radioactivity, a fundamental concept in nuclear medicine and radiation protection. The SI unit of radioactivity is a measure of the rate of decay or disintegration of radioactive substances.

**Why the Correct Answer is Right**
Radioactivity is measured in terms of the number of nuclei that undergo radioactive decay per unit time. The correct unit of measurement is the Becquerel (Bq), which is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. This unit is named after Henri Becquerel, the French physicist who discovered radioactivity. The Becquerel is derived from the SI base unit of time, the second, and is a measure of the rate of nuclear decay.

**Why Each Wrong Option is Incorrect**
**Option A:** The Gray (Gy) is a unit of absorbed radiation dose, not radioactivity. It measures the energy deposited per unit mass of tissue.
**Option B:** The Sievert (Sv) is a unit of equivalent dose, which takes into account the biological effect of radiation. It is not a direct measure of radioactivity.
**Option C:** The Roentgen (R) is a unit of exposure to ionizing radiation, commonly used in X-ray imaging. It is not a measure of radioactivity.

**Clinical Pearl / High-Yield Fact**
Remember that the Becquerel is the fundamental unit of radioactivity, and it is essential to distinguish it from other units of radiation measurement, such as the Gray and Sievert, which are used in different contexts.

**Correct Answer: C. Roentgen (R) is a unit of exposure to ionizing radiation, commonly used in X-ray imaging. It is not a measure of radioactivity.