Answer:
Pyramid, both a pyramid and a rectangle tapering toward the top result in a population that increases in size.
Step-by-step explanation:
In age structures, there are four main diagrams, depending if the population of men and women has a rapid growth, a slow growth, stable or a mixing of them.
In most parts of the world, population of different countries present a constant grow.
At the top of the graphic since is the representation of the old people, in a population, there will be less individuals with this characteristic. That is the reason of the decrease.
Hope this info is useful.