The correct answer is - b. developing countries.
Even though the developing countries have the youngest populations in the world, still the life expectancy is the lowest, and there's also still a very high percentage of deaths among the newly born babies. This is due to the poverty, low education, terrible living conditions, lack of fresh water which also is very often not suitable for drinking, the healthcare is on a very low level so diseases spread very easily and cause of lack of treatment very often result in death.