## 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 includes three main components:
1. Life expectancy at birth (reflecting a long and healthy life),
2. Expected years of schooling and mean years of schooling (reflecting knowledge),
3. Gross National Income (GNI) per capita (reflecting a decent standard of living).
Any factor not directly related to these three dimensions would not be included in the HDI.

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

## Why the Correct Answer is Right (Detailed for D)
- **Option D:** Not specified, but based on the process of elimination and understanding of HDI components, any option not directly related to life expectancy, education (knowledge), or GNI per capita would be correct.

## Clinical Pearl / High-Yield Fact
A key point to remember is that HDI is not solely an economic measure but also includes health and education, providing a more holistic view of a country's development.

## Correct Answer: D. Not specified