**Core Concept**
HU stands for Hounsfield Units, which is a standardized scale used to express the attenuation or density of various tissues within the body. This concept is crucial in computed tomography (CT) imaging, where HU values are used to differentiate between different types of tissues and diagnose various conditions.

**Why the Correct Answer is Right**
HU is a measure of the attenuation of X-rays by different tissues. The scale is calibrated such that water is set at 0 HU, and air is set at -1000 HU. This allows for the creation of a standardized reference point for comparing the density of various tissues. The HU values are calculated based on the attenuation coefficients of the tissues, which vary depending on the atomic number and density of the tissue. In CT imaging, HU values are used to differentiate between soft tissues, bones, and other structures within the body.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because HU is not a measure of the magnetic susceptibility of tissues, which is a characteristic measured in magnetic resonance imaging (MRI).
**Option B:** This option is incorrect because HU is not a measure of the chemical composition of tissues, although it can provide indirect information about tissue density and composition.
**Option C:** This option is incorrect because HU is not a measure of the electrical conductivity of tissues, which is an important characteristic in medical imaging and diagnosis.

**Clinical Pearl / High-Yield Fact**
It's essential for radiologists and clinicians to understand the HU scale to accurately interpret CT images and make informed decisions about patient care. A common pitfall is confusing HU values with other imaging characteristics, such as MRI signal intensity or ultrasound echogenicity.

**Correct Answer: C. Attenuation of X-rays by tissues.**