**Core Concept**
The sensitivity of a malignancy to radiotherapy depends on its inherent cellular characteristics, particularly the ability of tumor cells to undergo apoptosis or necrosis in response to ionizing radiation. Tumors with high mitotic activity, rapid growth rates, and defective DNA repair mechanisms tend to be more radiosensitive.

**Why the Correct Answer is Right**
Hodgkin's lymphoma is the most radiosensitive malignancy among the listed options. This is due to the presence of Langerhans giant cells, which are highly sensitive to radiation, and the lymphoma cells themselves, which have a high mitotic rate and undergo apoptosis in response to radiation-induced DNA damage. The combination of these factors makes Hodgkin's lymphoma an ideal candidate for radiotherapy.

**Why Each Wrong Option is Incorrect**
* **Option A:** Non-Hodgkin's lymphoma, while responsive to radiotherapy, is generally less sensitive than Hodgkin's lymphoma due to its heterogeneity and variable cellular characteristics.
* **Option B:** Osteosarcoma is a highly aggressive and malignant tumor, but it is relatively radioresistant due to its high DNA repair capacity and ability to adapt to radiation-induced damage.
* **Option D:** Melanoma is a highly aggressive and malignant tumor, but it is also relatively radioresistant due to its high DNA repair capacity and ability to undergo alternative cellular pathways in response to radiation-induced damage.

**Clinical Pearl / High-Yield Fact**
Hodgkin's lymphoma is an excellent example of a malignancy that is highly responsive to radiotherapy, and its treatment protocols often involve a combination of radiation and chemotherapy.

**Correct Answer:** C. Hodgkin's lymphoma