**Core Concept**
A linear accelerator is a medical device used in radiation therapy to deliver high-energy beams of radiation to cancerous tumors. It works by accelerating charged particles, such as electrons, to high speeds and then directing them at the tumor site.

**Why the Correct Answer is Right**
Linear accelerators produce megavoltage X-rays, which are a type of ionizing radiation. These high-energy X-rays are capable of penetrating deep into tissues, making them ideal for treating tumors that are located beneath the surface of the body. The linear accelerator achieves this by accelerating electrons to nearly the speed of light, which then collide with a metal target, producing the X-rays.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because _gamma knives_ are also used in radiation therapy, but they use cobalt-60 sources to produce gamma rays, which are different from the X-rays produced by linear accelerators.
**Option B:** This option is incorrect because _cobalt machines_ use cobalt-60 sources to produce gamma rays, which are not the same as the X-rays produced by linear accelerators.
**Option C:** This option is incorrect because _electron beam therapy_ can be delivered using a linear accelerator, but it is not the most accurate description of what is provided by the device.

**Clinical Pearl / High-Yield Fact**
Linear accelerators can produce a wide range of X-ray energies, from 4 to 25 MeV, which allows for precise control over the depth of penetration and the dose delivered to the tumor.

**Correct Answer: A. Megavoltage X-rays**