## **Core Concept**
The question pertains to the sensitivity of different parts of the body to radioactivity, which is a concept within the realm of radiation biology. Ionizing radiation can cause damage to living tissues, and different organs have varying levels of sensitivity to this type of radiation. The sensitivity is often related to the rate of cell division and the function of the cells within an organ.

## **Why the Correct Answer is Right**
The correct answer, **Bone Marrow (C)**, is highly sensitive to radioactivity because it is the site of blood cell production in the body. Bone marrow contains rapidly dividing hematopoietic stem cells, which are responsible for producing all types of blood cells, including white blood cells, red blood cells, and platelets. Rapidly dividing cells are more susceptible to the damaging effects of ionizing radiation, which can disrupt DNA replication and cell division, leading to cell death or mutations. This sensitivity makes bone marrow particularly vulnerable to radiation exposure.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While the gastrointestinal tract can be affected by radiation, leading to symptoms like nausea, vomiting, and diarrhea, it is not as highly sensitive to radioactivity as bone marrow in terms of the immediate and severe impact on cell production and organ function.
- **Option B:** The lungs can suffer from radiation pneumonitis and fibrosis following exposure to high doses of radiation, but like the gastrointestinal tract, they are not as immediately sensitive as bone marrow in terms of cell division and acute response to radiation.
- **Option D:** The liver has a more stable cell population and is less immediately sensitive to radiation compared to rapidly dividing tissues like bone marrow. While it can be affected by radiation, the liver's response is typically seen at higher doses and over a longer period.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is the **Law of Bergonie and Tribondeau (1906)**, which states that radiosensitivity is directly proportional to the reproductive activity of cells and inversely proportional to their degree of differentiation. This principle helps in understanding why rapidly dividing tissues, such as bone marrow, are highly sensitive to radiation.

## **Correct Answer:** C. Bone Marrow.