The correct answer is option 1, Indo-Gangetic Plains. Scheduled caste population in India refers to a social group that has been historically marginalized and faces caste-based discrimination. The Indo-Gangetic Plains, also known as the Indus-Ganga Plain, is a region in northern India that includes parts of Punjab, Haryana, Uttar Pradesh, Bihar, and West Bengal. This region is known for its dense population and agricultural productivity. It is also home to a significant concentration of the scheduled caste population, with a high proportion of scheduled caste communities residing in these states. These states have a long history of caste-based discrimination and segregation, which likely contributes to the higher concentration of scheduled caste population in this region.