The most true statement regarding on-reservation American Indian populations is B. They have the greatest access to tribal government services. This is because on-reservation American Indian populations are governed by tribal governments, which provide a range of services and programs to their members. These services may include healthcare, education, housing, social services, and economic development programs. Tribal governments are often better equipped to understand the needs of their communities and to provide culturally appropriate services and support. However, it is important to note that access to these services can still be limited due to factors such as funding constraints and geographic isolation.