The correct option is (B), (C), and (D) only.

Let us analyze each of the statements to understand why they are correct or incorrect:

Statement A is incorrect: Historically, Adivasis (tribes) were not always the oppressed groups they are in modern times. Before the colonial era and the expansion of the modern state, many tribal communities enjoyed significant autonomy, controlled vast forest resources, and held political power in their regions. The systematic oppression and marginalization of tribal communities largely accelerated during the colonial period through resource exploitation, forest laws, and land alienation. Therefore, stating that they were "always" the oppressed group is historically inaccurate.

Statement B is correct: Anthropologists and sociologists, including M.N. Srinivas, have documented how various tribal groups were gradually integrated into the broader Hindu social hierarchy. This integration often occurred through the process of "Sanskritisation," where tribes adopted the customs, rituals, beliefs, and lifestyle of upper-caste Hindus to claim a higher status in the social hierarchy.

Statement C is correct: Tribal societies were not completely isolated; they played a vital role in trade and economic networks. They occupied a specialized trade niche by collecting and trading valuable forest produce, timber, honey, wax, medicinal herbs, salt, and elephants (which were highly sought after by regional kingdoms for their armies).

Statement D is correct: Far from being passive victims, many hill and forest tribes historically exercised dominance or political leverage over the settled agricultural communities in the plains. Because of their knowledge of the difficult terrain and guerrilla tactics, tribes could raid plains settlements, and plains rulers often paid them tributes or tolls to secure peace and safe passage through forest routes.

Based on this historical analysis, statements B, C, and D are correct, making (B), (C), and (D) only the right answer.