Final answer:
The innovation that made nineteenth-century cities cleaner was sewage systems (option a). Transformations in urban planning, like the installation of modern sewer systems, notably after The Great Stink in London, significantly improved public health by reducing diseases associated with poor sanitation.
Step-by-step explanation:
The innovation that made nineteenth-century cities cleaner was a) Sewage systems. In response to the immense challenge of urban sanitation, cities such as London undertook monumental efforts to improve cleanliness and health. After incidents like The Great Stink of 1858, where the Thames River's overwhelming pollution essentially paralyzed the city, governments took drastic actions. The rebuilding of London's sewer system, overseen by Joseph Bazalgette between 1859 and 1875, is a prime example of such efforts. Not only was this a massive engineering achievement, but Bazalgette was also knighted for his contribution to public health.
Similar systems were established in other cities, and advancements in public health, such as access to clean water, dramatically reduced the incidence of diseases like cholera and typhoid fever. Public health acts, like the one passed in Britain in 1848, began to regulate the dumping of sewage, and other practices that contributed to public health issues. Although not all laws were immediately effective—such as those attempting to control industrial pollution—the establishment and enforcement of sanitation infrastructure marked a turning point in urban living conditions.