Answer:
D. the rapid growth of tenements and ghettos
Explanation:
In the early 19th century, less than 5% of humanity lived in cities. In the end of the 19th century, more than 20% of the world's population already lived in cities. (In the 21st century, three quarters of the inhabitants of developed countries live in cities.)
In the late 1800s the swelling of cities already had serious economic and social consequences  due to the rapidity of the urbanization process and the lack of urban infrastructures (transportation, energy, water, sewage, health and housing) to cater to all inhabitants. Most of these newly arrived workers would live in ghettos, slums or irregular areas.
