Final answer:
A population structure with a birthrate less than the death rate and a predominance of older individuals is best represented by a conical or inverted pyramid shape, reflecting a narrow base due to low birthrates and a wide top due to a higher proportion of post-reproductive individuals.
Step-by-step explanation:
The population age structure in which the birthrate is less than the death rate and where the post-reproductive group is the largest would be best represented by a graph with a conical or inverted pyramid shape.
In a population pyramid, a large base represents a high number of births, indicating rapid population growth, which is common in underdeveloped countries. Conversely, when the birthrate is low and the death rate is high, there are fewer younger individuals and a larger proportion of older, post-reproductive individuals. This leads to a narrower base and a wider top section of the pyramid.
This conical shape is often seen in developed countries that have low population growth or zero population growth. Populations with a higher ratio of post-reproductive individuals to young suggest that the population may be shrinking over time unless there is substantial immigration.