The correct answer is (b), Per Capita Income.
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. A country scores higher if it performs better on these indicators.
Life expectancy at birth is the average number of years that a newborn is expected to live if current mortality rates continue to apply. It is a good indicator of the overall health of a population.
Education is measured by the mean years of schooling and expected years of schooling. Mean years of schooling is the average number of years of schooling completed by people aged 25 and older. Expected years of schooling is the number of years of schooling that a child is expected to complete if current patterns of schooling continue.
Per capita income is the gross national income (GNI) per capita, calculated using the purchasing power parity (PPP) conversion factor. It is a measure of the average income of a country’s citizens.
The HDI is a useful tool for comparing the level of human development in different countries. It is also used to track progress over time. However, it has some limitations. For example, it does not take into account inequality within countries.