**Core Concept**
The facial index is a measurement used in anthropology and orthodontics to describe the shape of the human face. It is calculated as the ratio of the width of the face to its height, usually expressed as a percentage.

**Why the Correct Answer is Right**
The normal facial index ranges from 80 to 90%. This range is considered normal because it indicates a face that is slightly wider than it is tall, which is typical of most human populations. The facial index helps clinicians and researchers understand the underlying anatomy of the face and its relationship to the skull. A facial index within this range suggests a well-proportioned face that is neither too wide nor too narrow.

**Why Each Wrong Option is Incorrect**
**Option A:** A facial index of 65-75% is considered brachyfacial, meaning the face is too short and wide, which is not typical of most human populations.
**Option B:** A facial index of 75-80% is considered mesofacial, which is closer to the normal range but still on the narrower side.
**Option D:** A facial index of 100% would indicate a face that is as wide as it is tall, which is not a normal or typical facial proportion.

**Clinical Pearl / High-Yield Fact**
A high or low facial index can have implications for orthodontic treatment, as it may affect the alignment and spacing of teeth.

**✓ Correct Answer: C. 80 - 90%**