**Core Concept**
External Beam Radiotherapy (EBRT) is a type of radiation therapy where a beam of high-energy particles, such as X-rays, gamma rays, or protons, is directed at the tumor from outside the body. This technique is used to treat various types of cancer. The choice of radiation type depends on the specific cancer type, location, and patient factors.

**Why the Correct Answer is Right**
In EBRT, X-rays or gamma rays are commonly used due to their ability to penetrate deep into tissue. These radiation types interact with the tumor through the Compton effect, where the energy from the incident photon is transferred to the electrons in the tumor cells, causing damage to the DNA and ultimately leading to cell death. The correct answer is used in EBRT because of its penetration power and ability to interact with tumor cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is not commonly used in EBRT. While electrons are used in certain radiation therapy techniques, such as electron beam therapy, they are not typically used in EBRT.
* **Option B:** This option is incorrect because it is not a type of radiation used in EBRT. Protons are used in a specific type of radiation therapy called proton therapy, but it is not the same as EBRT.
* **Option D:** This option is incorrect because it is not a type of radiation used in EBRT. Neutrons are used in a specific type of radiation therapy called neutron therapy, but it is not the same as EBRT.

**Clinical Pearl / High-Yield Fact**
When treating cancer with EBRT, it's essential to consider the radiation type, dose, and fractionation to minimize damage to surrounding healthy tissues. This requires careful planning and execution to ensure effective tumor control while minimizing side effects.

**Correct Answer: C. X-rays.**