**Core Concept**
The radiosensitivity of a tumor refers to its susceptibility to damage caused by ionizing radiation, which can inhibit cell division and induce cell death. Tumors with a high degree of radiosensitivity are more likely to respond to radiation therapy, making them ideal candidates for this treatment modality. The radiosensitivity of a tumor is influenced by its cellular composition, particularly the presence of rapidly dividing cells.

**Why the Correct Answer is Right**
The correct answer is **Wilms tumor**. Wilms tumor, also known as nephroblastoma, is a type of kidney cancer that predominantly affects children. It is characterized by the presence of embryonal cells, which are highly sensitive to radiation. The embryonal cells in Wilms tumor are rapidly dividing and have a high degree of radiosensitivity, making radiation therapy a highly effective treatment option for this condition. The radiosensitivity of Wilms tumor is thought to be due to the presence of stem cells, which are highly sensitive to radiation-induced damage.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Rhabdomyosarcoma** is a type of soft tissue sarcoma that is relatively resistant to radiation therapy. While it can be treated with radiation, it is not as radiosensitive as Wilms tumor.
* **Option B:** **Ewing's sarcoma** is a type of bone cancer that is highly aggressive but not as radiosensitive as Wilms tumor. It can be treated with radiation, but it is often more resistant to radiation therapy than Wilms tumor.
* **Option C:** **Medulloblastoma** is a type of brain cancer that is highly aggressive and can be treated with radiation. However, it is not as radiosensitive as Wilms tumor and often requires more aggressive treatment approaches.

**Clinical Pearl / High-Yield Fact**
Wilms tumor is a classic example of a tumor that is highly responsive to radiation therapy, making it a key consideration in the treatment of pediatric kidney cancer. This tumor's high degree of radiosensitivity is due to the presence of embryonal cells, which are highly sensitive to radiation-induced damage.

**Correct Answer: D. Wilms tumor**