Which of the following is a main component of a city's population structure?

A city's population structure is fundamentally comprised of the demographics of its inhabitants, primarily focusing on age and sex distribution. This component provides valuable insights into the population's overall health, potential workforce, and trends related to birth rates, longevity, and migration patterns. For instance, a city with a large young population might indicate a vibrant, growing community, whereas an aging population could suggest challenges related to healthcare and social services.

While the other aspects mentioned do relate to the urban environment, they are not direct components of population structure. The types of architecture present reflect cultural and historical influences rather than demographics. Major industries give insight into the economic opportunities and employment landscape but do not describe the makeup of the population itself. Similarly, available transportation options are significant for mobility and accessibility within the city but do not depict who lives there or how many people belong to different age or gender categories.