If a country's age pyramid changes over 20 years from a triangular shape toward a more rectangular shape, which change is most likely?

Correct answer: Declines in both fertility and mortality with greater balance across age groups

A triangular age pyramid usually has a very broad base, showing a large share of children and relatively high fertility. It narrows quickly toward older ages when survival and life expectancy are lower. A more rectangular shape means that age groups are more evenly represented: the base is less wide and more people survive into middle and older ages. Over twenty years, this pattern is most commonly associated with falling fertility and falling mortality, producing greater balance across age groups.

Option A is correct because it explains both the narrower base and the wider survival pattern. Option B would tend to make the base broader, not rectangular. Option C would make the population structure less healthy and more sharply declining at older ages. Option D alone cannot normally explain a broad, sustained change across the whole age pyramid. The shape reflects demographic transition rather than only one migration event.