The age structure of population refers to:

The correct answer is Option (3) → the proportion of persons in the different age groups relative to the total population.

Option (1) the number of live births per 1000 women in the child-bearing age group — Incorrect. This is the fertility rate.

Option (2) the number of deaths of babies before the age of one year per 1000 live births — Incorrect. This is the infant mortality rate.

Option (3) the proportion of persons in the different age groups relative to the total population. — Correct. The age structure shows how the population is spread over age groups. It changes with the level of development and life expectancy: better health care means more people live into the older age groups.

Option (4) the number of females per 1000 males in a given area at a specified time period. — Incorrect. This is the sex ratio.