A nation's standard of living is determined by

a. the quantity of natural resources with which it is endowed.

b. factors and events that are beyond the nation's control.

c. the percentage of its gdp that is accounted for by government purchases.

d. the productivity of its workers

I would say that a nation's standard of living would depend primarily on the richness of its natural resources and also the productivity of its workers. If there are abundant valuable minerals and trees and fossil fuels and rich agricultural land, and its farmers and miners and loggers are hard working and productive then those are the prime reasons for a nation to have a high standard of living.

Choice (d) is the most correct. Productivity of the population and how much can be made (or served) is the biggest driver of standards of living. The more a country can output (and therefore, afford to bring in), the higher the overall standards will be.