Human Development Index comprises literacy rates, life expectancy at birth and

The Human Development Index HDI is a composite index used by the United Nations Development Programme to measure average achievement in three basic dimensions of human development. These dimensions are a long and healthy life measured by life expectancy at birth, knowledge measured by mean and expected years of schooling which covers literacy, and a decent standard of living.

The standard of living dimension is measured by Gross National Income GNI per capita at purchasing power parity. In the context of the options provided, National Income per head represents this economic component. While modern calculations specifically use GNI at purchasing power parity, traditional definitions in this format identify National Income per head as the standard measure for the third pillar of the index. Therefore, option D is the correct choice as it identifies the economic indicator required to calculate the index alongside health and education.