Ion which scatters X-ray most is –

**Core Concept**
The question is testing the principle of the Compton scattering effect, which is a fundamental concept in radiology. Compton scattering is the scattering of a photon by a free charged particle, typically an electron. This phenomenon is crucial in understanding how X-rays interact with matter.

**Why the Correct Answer is Right**
In the context of X-ray scattering, Compton scattering is the dominant process that occurs when X-rays collide with electrons in the material. The scattered X-ray is deflected in a new direction due to the collision, resulting in a loss of energy. This process is characterized by the scattering of X-rays by free electrons, which is the primary mechanism of X-ray scattering in materials. The Compton effect is described by the Compton scattering formula, which relates the wavelength of the scattered X-ray to the scattering angle.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Compton scattering is the correct process that occurs when X-rays interact with free electrons in a material.
* **Option B:** This option is incorrect because Rayleigh scattering, which is the scattering of light by particles much smaller than the wavelength of the light, is not the dominant process in X-ray scattering.
* **Option C:** This option is incorrect because photoelectric absorption is a different process that occurs when X-rays are absorbed by a material, rather than scattered.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Compton scattering is the primary mechanism of X-ray scattering in materials, and it's responsible for the characteristic "haze" or "scatter" seen in radiographic images.

**Correct Answer:** C. Photoelectric effect