These apartment-style buildings were called "Insulae", and housed most of ancient Rome's urban population, which consisted of the low and middle classes. These buildings were usually cramped, small, three to five stories high and housed from 30 to 50 people. And although they became a solution for the densely populated cities, it later became a major problem as many of the buildings were not well constructed and were prone to collapse or catch fire.
I believe you're asking which of those options is correct and here's what I'd say it is: Most residents of ancient Rome lived in apartment-style buildings, they were called "insulas."
