**Core Concept**
The question requires knowledge of the radiosensitivity of various tumors, which is a critical concept in radiation oncology. Radiosensitivity refers to the susceptibility of a tumor to be killed or damaged by radiation therapy. Different tumors have varying levels of radiosensitivity, and understanding this concept is essential for selecting effective treatment modalities.

**Why the Correct Answer is Right**
The most radiosensitive tumor among the options is **Wilm's tumor (Nephroblastoma)**. This is because Wilm's tumor is highly sensitive to radiation, and radiation therapy is often used as a primary treatment modality. The tumor's radiosensitivity is thought to be due to its rapid growth rate and high mitotic activity, making it more susceptible to damage from radiation. The radiation-induced damage to the tumor cells' DNA leads to cell death, thereby controlling the tumor.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Rhabdomyosarcoma** is a soft tissue sarcoma that is not as radiosensitive as Wilm's tumor. While it can be treated with radiation, it requires higher doses and may not be as responsive as Wilm's tumor.
* **Option B:** **Ewing's sarcoma** is a malignant bone tumor that is radiosensitive, but its radiosensitivity is not as high as that of Wilm's tumor. Ewing's sarcoma often requires a combination of radiation and chemotherapy for effective treatment.
* **Option D:** **Osteosarcoma** is a malignant bone tumor that is relatively radioresistant. It is often treated with surgery and chemotherapy, with radiation therapy used in selected cases.

**Clinical Pearl / High-Yield Fact**
Wilm's tumor is the most common renal malignancy in children, and its radiosensitivity is one of the reasons why radiation therapy is often used as a primary treatment modality. It is essential to remember that Wilm's tumor is highly sensitive to radiation, making it an ideal candidate for radiation therapy.

**Correct Answer:** A. Wilm's tumor (Nephroblastoma)