Final answer:
The single most important determinant of cross-country differences in income is the difference in technology / productivity. Technological advancements and higher productivity allow countries to produce more output with the same amount of resources, leading to higher incomes.
Step-by-step explanation:
The single most important determinant of cross-country differences in income is the difference in technology / productivity. While factors such as savings rate, population growth rate, and government spending can also influence income differences to some extent, technology and productivity have a greater impact on economic growth and income levels. Technological advancements and higher productivity allow countries to produce more output with the same amount of resources, leading to higher incomes.
For example, countries that invest in research and development, adopt new technologies, and have efficient production processes tend to have higher incomes compared to countries with outdated technology and low productivity. Therefore, the answer to the question is d. technology / productivity.