**Core Concept**
In computed tomography (CT), attenuation values are a measure of the reduction in intensity of X-ray beams as they pass through the body. Hounsfield units (HU) are a standardized scale used to express these attenuation values, with water serving as the reference point.

**Why the Correct Answer is Right**
An attenuation value of '0' HU corresponds to water because it is the reference material used to calibrate CT scanners. Water has a defined attenuation coefficient, and all other materials are compared to it on the Hounsfield scale. This allows for accurate measurement and representation of tissue densities across the body. As a result, water is assigned a value of 0 HU, serving as the zero point for the scale.

**Why Each Wrong Option is Incorrect**
**Option B:** Air has a much lower attenuation coefficient than water, resulting in a negative Hounsfield value (typically around -1000 HU). This is because air is less dense and allows more X-ray photons to pass through.
**Option C:** Very dense bone structures have a high attenuation coefficient, resulting in high positive Hounsfield values (often above 1000 HU). This is because bone is highly dense and absorbs many X-ray photons.
**Option D:** Fat has a lower attenuation coefficient than water but is not as low as air. As a result, fat typically has a Hounsfield value around -50 to -100 HU.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the Hounsfield scale is linear only in the range of -1000 to +1000 HU. Outside of this range, the scale becomes non-linear, and values are not directly proportional to the attenuation coefficient.

**✓ Correct Answer: A. Water**