**Core Concept**
Intra-operative radiotherapy (IORT) is a technique that delivers high doses of radiation directly to a tumor bed during surgery, minimizing exposure to surrounding healthy tissues. This approach is particularly useful in treating tumors with a high risk of recurrence, such as sarcomas and breast cancers.

**Why the Correct Answer is Right**
The correct answer employs a therapeutic mode that involves the use of electrons to deliver radiation directly to the tumor bed. Electron therapy is ideal for IORT because it allows for precise targeting of the tumor site, with minimal penetration into surrounding tissues. This reduces the risk of radiation-induced damage to healthy tissues and organs. The electron beam is carefully calibrated to match the dimensions of the tumor bed, ensuring that the radiation dose is concentrated in the target area.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the use of electrons, which is a critical aspect of intra-operative radiotherapy. While electron therapy is used in other cancer treatments, it is not typically employed in IORT.
**Option B:** This option is incorrect because it refers to external beam radiation therapy (EBRT), which involves delivering radiation from outside the body. EBRT is not suitable for IORT, which requires direct access to the tumor site during surgery.
**Option C:** This option is incorrect because it refers to brachytherapy, which involves placing radioactive sources directly into or near the tumor. While brachytherapy is a form of internal radiation therapy, it is not the same as IORT, which involves delivering radiation during surgery.

**Clinical Pearl / High-Yield Fact**
When performing IORT, it's essential to consider the energy of the electron beam and the thickness of the tissue to be treated. This ensures that the radiation dose is delivered effectively and safely, minimizing the risk of radiation-induced damage to surrounding tissues.

**Correct Answer: C. Electron therapy.