The correct answer is D) sewer systems.
The other options of the question were A) roads B) aqueducts. C) public baths.
What helped resolve the issue of sanitation in overcrowded cities in Rome were sewer systems.
The Romans were good engineers and built sewer systems to help the sanitation of their cities. Dark water flowed using this engineering system. Different channels directed the water to a central sewage system that deposited waters out in rivers. For clean water, ancient Rome architects built aqueducts that served most of the cities.