**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 is a widely used metric to measure the well-being of a nation.

**Why the Correct Answer is Right**
The HDI focuses on three key dimensions of human development:

* **Life expectancy at birth**: This measures the average number of years a person is expected to live free from disability and illness.
* **Expected years of schooling**: This measures the average number of years of education that a child can expect to receive.
* **Gross National Income (GNI) per capita**: This measures the total income earned by a country divided by its population.

**Why Each Wrong Option is Incorrect**
**Option A:** Not mentioned in the context of HDI.
**Option B:** Not mentioned in the context of HDI.
**Option C:** Not mentioned in the context of HDI.

**Clinical Pearl / High-Yield Fact**
The HDI is a useful tool for policymakers and researchers to identify areas of improvement in human development. It is calculated by the United Nations Development Programme (UNDP) and is used to track progress towards the Sustainable Development Goals (SDGs).

**Correct Answer:** C.