Alert - correct answer should be 2: Gross Domestic Product per head at real purchasing power.

The Human Development Index (HDI) is a statistic composite index including three factors: literacy rate, life expectancy at birth, and the standard of living which is calculated using gross domestic product per capita at purchasing power parity (PPP).

Option 1, gross domestic product per head in US dollars, is not used because it doesn`t balance the difference in cost of living across different countries.

Option 3, gross national product (GNP) in US dollars, also is not considered as the GNP includes the economy of a country`s residents and businesses, regardless of the countries where they function, which could distort the assessment of the standard of living.

Finally, option 4, national income per head in US dollars, is incorrect. Although similar to GDP, it doesn`t include income from capital depreciation or indirect business taxes. For the purpose of HDI, GDP at PPP is a more accurate representation of standard of living.