**Core Concept**
The question is testing the student's knowledge of the radiosensitivity of bone tumors. Radiosensitivity refers to the susceptibility of cells or tissues to damage from ionizing radiation. In cancer treatment, radiosensitivity is a critical factor in determining the effectiveness of radiation therapy.

**Why the Correct Answer is Right**
The correct answer is Ewing's sarcoma. Ewing's sarcoma is a type of malignant bone tumor that is highly radiosensitive. This means that it responds well to radiation therapy, making it a good candidate for this treatment option. The radiosensitivity of Ewing's sarcoma is due to the high rate of cell division and the presence of specific molecular markers that make the cells more vulnerable to radiation damage.

**Why Each Wrong Option is Incorrect**
* **Option A:** Osteosarcoma is a type of bone tumor, but it is not as radiosensitive as Ewing's sarcoma. Osteosarcoma cells are more resistant to radiation damage due to their slower rate of cell division and the presence of protective mechanisms that help them survive radiation exposure.
* **Option B:** Chondrosarcoma is another type of bone tumor, but it is relatively radioresistant. Chondrosarcoma cells are less sensitive to radiation damage due to their slow rate of cell division and the production of extracellular matrix that provides protection against radiation.
* **Option C:** Multiple myeloma is a type of bone marrow cancer, but it is not a primary bone tumor. While radiation therapy may be used to treat multiple myeloma, it is not the most radiosensitive bone tumor.

**Clinical Pearl / High-Yield Fact**
Remember that Ewing's sarcoma is a highly aggressive bone tumor that often presents in young adults. It is characterized by its rapid growth, pain, and swelling in the affected bone. Radiation therapy is a key component of treatment for Ewing's sarcoma, and it is often used in combination with chemotherapy and surgery.

**Correct Answer:** E. Ewing's sarcoma.