**Core Concept:** Filter used in X-ray tube is a component that reduces the high energy photons from the X-ray tube to a safe level for exposure. This prevents excessive radiation exposure to the patient and the medical personnel handling the X-ray equipment. The filters are usually made up of materials that absorb the high-energy photons and convert them into lower energy photons.

**Why the Correct Answer is Right:** The correct answer is B (Aluminium filter), because aluminium is a suitable material for filtering out high-energy photons due to its high atomic number (Z=13) and high density. High atomic number materials absorb more X-rays as they have a higher number of protons that can collide with incoming X-rays, depriving them of energy. Aluminium filters can effectively reduce the energy of the X-rays, making them safe for human exposure.

**Why Each Wrong Option is Incorrect:**

A. Graphite (carbon): Graphite has a lower atomic number (Z=6) compared to aluminium (Z=13), making it less effective in absorbing X-rays.
C. Copper: Copper has a similar atomic number (Z=29) to aluminium, but its density is lower, making aluminium an even better choice for filtering X-rays.
D. Lead: While lead has a higher atomic number (Z=53) than aluminium, it is denser, which makes aluminium a better choice as it can achieve similar filtration with less lead, reducing the weight and cost of the X-ray equipment.

**Clinical Pearl:**

The choice of X-ray tube filters is crucial in radiography as it ensures safe radiation exposure for patients and healthcare workers while maintaining the quality of X-ray images. Aluminium filters are commonly used due to their effectiveness, cost-effectiveness, and ease of handling. Understanding the role of filters in X-ray protection is essential for medical professionals involved in radiography procedures.