Which is most challenging for developing countries?

large aging populations
economic and employment issues
social and environmental issues
outsourcing of jobs to nearby areas

All of the mentioned problems are somewhat related to developing countries, but the biggest issue is deffinitely economic and employment issues. Developing countries do not have stable market and are often transitioning from state owned to privately owned companies. They are also struggling to atract investitors, because big global companies do not want to engage with the risks of an unstable market. Due to this, job market is also very low and the unemployment rate is high. Often times, minimum wage is lower than the average standard.

The answer is "large aging populations".

Population aging—the expansion of the offer of more established people in a general public because of ripeness decays and rising future—is an irreversible worldwide pattern with sweeping financial and socio-political outcomes.  

Aging populations represent a challenge to the financial and macroeconomic dependability of numerous social orders through expanded government spending on annuity, human services, and social advantages programs for the elderly.

answer; the confederacy could afford to move a large number of prisoners hundreds of miles but could not afford to fee and house the prisoners;