## **Core Concept**
The Human Development Index (HDI) is a composite statistic of life expectancy, education, and per capita income indicators, which are used to rank countries into four tiers of human development. It was introduced by the United Nations Development Programme (UNDP) to assess and compare the level of development across countries.

## **Why the Correct Answer is Right**
The HDI is composed of three main components:
- Life expectancy at birth (reflecting health and longevity),
- Expected years of schooling and mean years of schooling (reflecting education),
- Gross National Income (GNI) per capita (reflecting standard of living).

## **Why Each Wrong Option is Incorrect**
- **Option A:** Life expectancy is indeed one of the components of HDI, reflecting the health and longevity aspect.
- **Option B:** Expected and mean years of schooling are also components, reflecting the education aspect.
- **Option C:** Gross National Income (GNI) per capita is a component, reflecting the standard of living.
- **Option D:** Literacy rate, while an important indicator of education and often used in various indices, is not a direct component of the HDI. Instead, HDI uses expected years of schooling and mean years of schooling.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that HDI does not directly include indicators like literacy rate, infant mortality rate, or poverty rate, even though these are crucial for assessing development. The focus is on longevity, education, and income.

## **Correct Answer:** D. Literacy rate.