Which demographic change refers specifically to shifts in the population structure over a period?

The correct choice highlights the concept of demographic transition, which refers to the shifts in population structure over time, particularly relating to the changes in birth rates and death rates. This transition is often characterized by a move from high birth and death rates to lower birth and death rates as a society develops economically and socially. The demographic transition model illustrates stages that societies typically pass through, reflecting changes in population growth and structure.

Fertility change specifically focuses on variations in birth rates but does not encompass the broader shifts that characterize demographic transition. Population density refers to the number of individuals per unit area, providing a snapshot of how clustered or spaced populations are but not directly addressing changes in the population structure itself. Social mobility pertains to the ability of individuals or groups to move within a social hierarchy, which does not directly relate to changes in population metrics over time. Overall, demographic transition captures the comprehensive evolution of population characteristics, making it the most appropriate answer.