## **Core Concept**
The Human Development Index (HDI) is a composite index measuring average achievement in three basic dimensions of human development: a long and healthy life, knowledge, and a decent standard of living. It is used to assess and compare the level of development across different countries.

## **Why the Correct Answer is Right**
The HDI is composed of three main indicators:
- Life expectancy at birth (reflecting a long and healthy life)
- Expected years of schooling and mean years of schooling (reflecting knowledge)
- Gross National Income (GNI) per capita (reflecting a decent standard of living)

## **Why Each Wrong Option is Incorrect**
- **Option A:** Life expectancy is indeed one of the components, reflecting the dimension of a long and healthy life.
- **Option B:** Expected and mean years of schooling are part of the HDI, representing the knowledge dimension.
- **Option C:** Gross National Income (GNI) per capita is also a component, representing the decent standard of living dimension.

## **Why D is Correct but not explicitly listed:**
Since the actual options (A, B, C, D) are not provided in the query, we infer based on common knowledge that HDI includes life expectancy, education (which encompasses expected and mean years of schooling), and GNI per capita. Typically, **mortality rate**, while related to life expectancy, or other factors not directly listed (like **poverty rate**, **inequality**, etc.) might not be directly included as a separate component.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while HDI provides a broad view of human development, it does not directly measure **inequality**, **environmental sustainability**, or **specific health indicators** beyond life expectancy.

## **Correct Answer:** D. (Assuming D represents an option not directly included in the HDI calculation like inequality, environmental factors, etc., which are important but not part of the three core dimensions.)