Answer: The correct answer is " They often lived in overcrowded neighborhoods with poor sanitation and disease." The case Study mentioned how the population of New York tripled years after the civil war with it mainly being immigrants. Due to this huge unexpected spike in New York they often had to live in crowded apartments and rooms. Since the case study also mentioned that New York City has about 26,403 people per square mile, it adds to the sheer numbers of how overcrowded it was.
So the answer was " They often lived in overcrowded neighborhoods with poor sanitation and disease".