**Core Concept**
Centiles are a type of measure used in statistics to divide a dataset into equal-sized groups. They are a way to express the position of a value within a dataset, with the 50th centile (median) being the middle value. Centiles are useful in medicine for understanding the distribution of various health metrics, such as height, weight, or blood pressure.

**Why the Correct Answer is Right**
The correct answer is **C. 100 equal parts**. Centiles divide data into 100 equal parts, with each part representing a specific percentage of the dataset. For example, the 10th centile represents the value below which 10% of the data falls. This allows for a more nuanced understanding of data distribution and can be particularly useful in clinical settings where understanding the range of normal values is crucial.

**Why Each Wrong Option is Incorrect**
* **Option A:** Centiles do not divide data into 50 equal parts. While the median (50th centile) is an important value, centiles are not defined by dividing data into 50 parts.
* **Option B:** Centiles do not divide data into 25 equal parts. While quartiles (25th, 50th, and 75th centiles) are an important part of data analysis, centiles are not defined by dividing data into 25 parts.

**Clinical Pearl / High-Yield Fact**
When interpreting centiles, it's essential to remember that the 5th and 95th centiles represent the range of values within which 90% of the data falls. This can be a useful way to understand the normal range of values for a particular metric.

**Correct Answer:** C. 100 equal parts