Final answer:
Vaccination played a crucial role in reducing the incidence and virulence of tuberculosis in the early twentieth century. Other factors like improved sanitation and nutrition also contributed to the decline in tuberculosis cases.
Step-by-step explanation:
The correct answer to the question is b) Vaccination.
Vaccination played a crucial role in reducing the incidence and virulence of tuberculosis in the early twentieth century. Vaccines stimulate the immune system to produce specific antibodies against the bacteria that cause tuberculosis, preventing infection or reducing the severity of the disease. This has led to a significant drop in the incidence of tuberculosis and the elimination of other infectious diseases, such as smallpox.
Additionally, other factors like improved sanitation and nutrition also contributed to the decline in tuberculosis cases during that period.