Assertion (A) is true: Tuberculosis has historically been a leading cause of adult mortality in India, often killing more adults than many other single diseases for a significant period. Reason (R) is true: TB disproportionately affects individuals in the economically productive age group, typically between 15 and 59 years. This age group constitutes a large portion of the adult population. The fact that the disease primarily targets this large adult demographic explains why it accounts for a high number of adult deaths compared to other diseases that might affect different age groups or have lower fatality rates in adults.