If a country's birth rate declines rapidly, in which part of the population pyramid is the change most likely to appear first?

Correct answer: Base

A population pyramid shows the size of different age groups. The birth rate directly determines the size of the youngest groups because these groups consist of children born most recently. If births decline rapidly, the new cohorts entering the bottom of the pyramid are smaller. The base therefore begins to narrow first. Older cohorts were born under earlier conditions, so their numbers cannot change immediately because of the current fall in births.

Option A is correct because it identifies the base as the part most directly affected. The topmost part changes mainly through survival and life expectancy, not through an immediate birth-rate decline. The middle section represents people born earlier and will respond later as smaller cohorts move upward through the age structure. All parts will not change equally or at the same time, so D is incorrect. The effect may become clearer across successive age groups over several years.