**Core Concept**
The Human Development Index (HDI) is a composite measure of national development, encompassing three key dimensions: life expectancy at birth, expected years of schooling, and Gross National Income (GNI) per capita. It aims to provide a more comprehensive understanding of a country's development beyond mere economic indicators.

**Why the Correct Answer is Right**
The HDI takes into account three main components:
1. **Life expectancy at birth**, which reflects the health and well-being of a population.
2. **Expected years of schooling**, indicating the quality and accessibility of education.
3. **Gross National Income (GNI) per capita**, representing the economic prosperity of a nation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Not mentioned in the provided options.
* **Option B:** Not a correct option.
* **Option C:** Not a correct option.

**Clinical Pearl / High-Yield Fact**
The HDI is a useful tool for policymakers and researchers to evaluate the overall well-being and development of a country. It helps identify areas for improvement and track progress over time.

**Correct Answer:** D.