**Core Concept**
The principle used in radiotherapy involves the ionization of water molecules in cells, leading to the formation of reactive oxygen species (ROS) that damage DNA and ultimately cause cell death. This process is known as radiation-induced apoptosis.

**Why the Correct Answer is Right**
The correct principle is based on the linear energy transfer (LET) of radiation, which is the rate at which energy is deposited along the path of the radiation. In radiotherapy, high-energy radiation is used to target cancer cells, causing DNA damage and disrupting cellular function. The ionization of water molecules in cells leads to the formation of ROS, which in turn cause DNA strand breaks, base damage, and other forms of DNA damage. This ultimately leads to cell death through apoptosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the principle used in radiotherapy. While radiation therapy does involve the use of high-energy radiation, it is not a principle used in the treatment.
**Option B:** This option is incorrect because it is a principle used in chemotherapy, not radiotherapy. Chemotherapy involves the use of medications to target cancer cells, whereas radiotherapy involves the use of high-energy radiation.
**Option C:** This option is incorrect because it is a principle used in surgical oncology, not radiotherapy. Surgical oncology involves the removal of tumors and cancerous tissue through surgical means, whereas radiotherapy involves the use of high-energy radiation.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that radiation-induced damage to DNA can lead to genetic mutations and increased risk of secondary cancers. This is why radiation therapy is typically used in combination with other treatments, such as surgery and chemotherapy, to minimize the risk of long-term complications.

**Correct Answer:** C.