(C) Gross Domestic Product
GDP (per capita income) is the most commonly used indicator to measure and compare the level of development of different countries.
Source: How to Compare Different Countries or States?, Chapter 1
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The passage states that income (per capita income / GDP) is considered "one of the most important attributes" for comparing countries' development. The World Bank uses per capita income as the main criterion for classifying countries. Primary and Secondary Production are not development indicators; NDP, while related, is not the standard measure cited. Examiners expect you to identify GDP as the key income-based development indicator.