Concept:Increasing returns to population mean that extra people cause output to rise more than proportionally.Explanation:A country experiences increasing returns when its population is still below the optimum level.This means the available resources and capital are not yet fully used.Adding more workers leads to a more than proportional increase in total output.Such a country is therefore under-populated rather than overpopulated.Overpopulation is linked with diminishing returns, not increasing returns.Answer:D. under-populated