Which factor contributed MOST to the rise of inner-city overcrowding in the late nineteenth century?

The most significant factor contributing to inner-city overcrowding in the late nineteenth century was the massive influx of immigrants seeking better economic opportunities. As industries expanded and jobs became available in urban areas, millions of people, particularly from Europe, moved to cities in search of work. This rapid population growth strained housing availability, leading to overcrowded living conditions in tenements and slums. Additionally, rural residents migrating to cities for jobs further exacerbated the situation.