Final answer:
The numbers between the bars in a population pyramid represent the age classes in 5-year ranges. These horizontal bars depict the population size or percentages of males and females within each age group, illustrating the age-sex structure of the population.
Step-by-step explanation:
The numbers between the bars in the middle of population pyramids act as the y-axis, and they represent the age classes, typically in 5-year ranges. Thus, the correct answer to the question is option 2) the age classes, in 5-year ranges. Each horizontal bar on the population pyramid represents the size or percentage of the population within that specific age range, with females usually displayed on the right and males on the left. The bars may show the total population in each age group either in absolute numbers or as percentages of the total population.
The bars become narrower from younger to older ages in a population pyramid due to different factors like declining birth rates, increase in mortality with age, and other demographic trends. As a population ages, there are generally fewer individuals in each successive age cohort. This visual representation helps us understand the age-sex structure of a population which is crucial for planning and allocating resources, among other demographic analysis.