This does not appear to be the sort of question that can be answered by filling one word into a blank. Instead, it looks like something that needs at least a short answer. One way to end this sentence would be to fill in the words “many urban problems that had not previously existed.”
When industrialization brought people to the cities, the cities were really not ready for them. There was not enough housing for the waves of migrants. In addition, many of the people coming to the cities were rather poor. They had been poor peasants when they lived in the countryside and they had been forced off the land for a variety of reasons. Thus, they were poor people who were swarming into cities that were not ready for them.
This led to a number of problems. First, it led to the creation of slums with very poor housing. The poverty of the migrants meant that they could typically not afford decent housing, forcing them to live in extremely unpleasant conditions. Second, it led to serious health problems. As people packed into cities with poor sanitation and little understanding of how disease spread, the conditions were ripe for diseases to spread. A famous example of this is the London cholera epidemic of 1854. Finally, this rapid urbanization led to an increase in crime. Not all of the people who came to the cities were able to find work. Not everyone who could find work was able to find a job that paid well enough to support a family. Furthermore, the social control mechanisms of small country communities simply did not exist in cities. For these reasons, the slums were dangerous places where crime was quite common. All of these were problems that came about because of the rapid urbanization that came along with industrialization.