When determining the standard of living in a country, the most practical and reliable indicator among the options provided is:
C. per capita GDP
Per capita GDP gives a more accurate representation of the average economic output per person in a country, which helps to assess the living standards of individuals within that country. It accounts for the population size and provides a clearer picture of economic well-being than total GDP. Other indicators, such as CPI or unemployment rate, may also provide useful information but do not directly measure the economic conditions experienced by individuals as effectively as per capita GDP.