The correct option is (b) Income.
For comparing countries, their income is considered to be one of the most important attributes by the World Bank.
The textbook (Chapter 1, How to Compare Different Countries or States?) explicitly states: "For comparing countries, their income is considered to be one of the most important attributes." The World Bank uses per capita income as the main criterion to classify countries as high-income, middle-income, or low-income. Students must not confuse this with the UNDP's criterion, which additionally includes health and education (HDI).