Which description best fits an expanding age pyramid?

Correct answer: Broad base tapering rapidly upward

An expanding age pyramid represents a population with many children and young people compared with older groups. A relatively high birth rate creates a broad youngest group. As age increases, groups usually become narrower because of deaths and, in some cases, migration. The pyramid therefore tapers upward toward the oldest ages.

Option A is correct because a broad base and rapidly narrowing upper part are the typical visual signs of an expanding population. A narrow base suggests low recent fertility and possible ageing, while an almost equal width represents a more stable structure. A population cannot normally consist only of middle-aged people, so option D is not an appropriate description.