Which is most challenging for developing countries?
large aging populations
economic and employment issues
social and environmental issues
outsourcing of jobs to nearby areas

The answer is "social and environmental issues".
In a developing country, populations are generally fast growing, but have a lower standard of living, thus a lower life expectancy.
These areas are also sources of cheap labor for developed countries, so jobs are outsourced TO these countries, not from them.
Finally, economic issues could be prevalent, but these societies usually have lots of jobs available. Hope this helps!