**Core Concept**
The Human Development Index (HDI) is a composite measure of national development, encompassing three fundamental dimensions: a long and healthy life, knowledge, and a decent standard of living. It is used to rank countries based on their development progress.

**Why the Correct Answer is Right**
The HDI is calculated using three key indicators:
* Life expectancy at birth (LEB), reflecting a long and healthy life.
* Expected years of schooling (EYS) and mean years of schooling (MYS), representing knowledge.
* Gross National Income (GNI) per capita, adjusted for purchasing power parity (PPP), indicating a decent standard of living.

**Why Each Wrong Option is Incorrect**
**Option A:** HDI is not measured by income inequality, although it may be related to overall economic development.
**Option B:** Education level, while relevant, is not the sole component of HDI; rather, it is a part of the knowledge dimension.
**Option C:** Healthcare expenditure is not a direct component of HDI; instead, life expectancy at birth is used as an indicator of health.

**Clinical Pearl / High-Yield Fact**
The HDI is a useful tool for comparing development across countries, but it has limitations in capturing nuances of national progress. It does not account for inequality within countries or the distribution of resources.

**Correct Answer:** D.