Figure $\mathbf{A}$ is a 'pyramid' shaped age pyramid. In this figure, the base, i.e., pre-reproductive stage is very large as compared with the reproductive and post-reproductive stages of the population. This type of age pyramid indicates that the population increase would be rapid.
Figure $\mathbf{B}$ is an 'inverted bell' shaped pyramid. In this figure the pre-reproductive and reproductive stages are same. Stable population is represented by this type of age structure.
Figure $\mathbf{C}$ is 'urn' shaped pyramid. In this figure, the prereproductive and reproductive stages are less compared to the post-reproductive stages of this population. In this population more older people are present. This type of age structure indicates declining population.