**Core Concept**
The Human Development Index (HDI) is a composite measure used to assess a country's well-being by considering factors such as life expectancy, education, and income. It provides a comprehensive view of a nation's development, beyond mere economic indicators.

**Why the Correct Answer is Right**
The HDI is a weighted average of three key dimensions: life expectancy at birth, expected years of schooling, and gross national income (GNI) per capita. These dimensions are chosen because they are critical indicators of a population's health, education, and economic opportunities. The GNI per capita is used as a proxy for income, while expected years of schooling and life expectancy reflect the country's ability to provide its citizens with access to quality education and healthcare.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the HDI does not solely focus on economic indicators. While income is an important aspect, it is just one of the three dimensions used to calculate the HDI.

**Option B:** This option is incorrect because the HDI does not directly measure poverty rates. Although income is an important component of the HDI, it does not provide a comprehensive picture of a country's poverty levels.

**Option C:** This option is incorrect because the HDI does not consider environmental sustainability as a primary factor. While environmental concerns are important, the HDI focuses on human well-being, education, and income.

**Clinical Pearl / High-Yield Fact**
The HDI is a valuable tool for policymakers and researchers to evaluate the effectiveness of development programs and make data-driven decisions. It helps identify areas of improvement and provides a framework for measuring progress towards sustainable development goals.

**Correct Answer: D. Gross national income (GNI) per capita.