The correct option is 1 only
Explanation
The term "Adivasis" refers to the original inhabitants or indigenous communities of India, constitutionally recognized as Scheduled Tribes (STs). They are characterized by distinct cultures, geographical isolation, and social systems that differ significantly from the mainstream social structure.
Statement 1 is Correct:
Historically, Adivasis have inhabited the hilly and forested regions of the subcontinent. They led a relatively secluded life, maintaining a symbiotic relationship with nature. This geographical isolation allowed them to preserve their distinct languages, customs, and religious practices independent of the empires and civilizations in the plains.
Statement 2 is Incorrect:
Adivasi societies are traditionally distinct from the mainstream Hindu social order. They are generally marked by the absence of the rigid caste hierarchy (Varna/Jati system). While there has been some interaction and acculturation over centuries, they were not "fully integrated" into the caste hierarchy; they maintained their own egalitarian social structures.
Statement 3 is Incorrect:
According to the Census of India 2011, Scheduled Tribes constitute 8.6% of the total Indian population. The figure of 16.6% corresponds to the population of Scheduled Castes (SCs).
Key Takeaway:
Scheduled Tribes (Adivasis) constitute 8.6% of India's population (2011 Census) and are historically characterized by their isolation and lack of a rigid caste-based social hierarchy.