## **Core Concept**
The unit of radioactivity is a measure of the amount of radioactive material present and its rate of decay. Radioactivity is quantified based on the number of nuclear disintegrations per unit time. The **SI unit** for radioactivity is the **becquerel (Bq)**, which represents one nuclear disintegration per second.

## **Why the Correct Answer is Right**
The becquerel (Bq) is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. This unit directly measures the rate of nuclear disintegrations, making it a direct measure of radioactivity. The becquerel is a fundamental unit in nuclear physics and medicine, used to express the activity of radioactive sources, including those used in diagnostic and therapeutic procedures.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is blank and does not provide a recognizable unit of measurement for radioactivity.
- **Option B:** This option is also blank and does not correspond to a valid unit of radioactivity.
- **Option D:** This option is blank as well and does not represent a standard unit for measuring radioactivity.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that in clinical and medical contexts, radioactivity is often expressed in terms of becquerels (Bq) or curies (Ci), where 1 Ci = 3.7 × 10^10 Bq. Understanding the units of radioactivity is crucial for safe handling and application of radioactive materials in medicine.

## **Correct Answer:** B.