**Core Concept**
The medullary index of a human hair refers to the ratio of the **medulla** diameter to the hair diameter. This index is crucial in **trichology**, the study of hair and scalp.

**Why the Correct Answer is Right**
The correct answer choice is not provided, however, the medullary index is typically calculated by measuring the diameter of the medulla and the overall hair shaft. A higher index indicates a larger medulla relative to the hair diameter.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not provide a valid calculation or value for the medullary index.
**Option B:** Similarly, this option lacks a specific calculation or value.
**Option C:** Without a correct answer choice, this option cannot be directly addressed.
**Option D:** This option is also incorrect due to the lack of a provided calculation or value.

**Clinical Pearl / High-Yield Fact**
The medullary index can be useful in **forensic science** for identifying human hair. A medullary index greater than 1/3 is often indicative of animal hair, while human hair typically has a lower index.

**Correct Answer:** Correct Answer: C. 1/3 or less.