Regionalism in India can be understood with reference to:

Ethnicity

The correct answer is Option (3) → Ethnicity

NCERT mentions that "After Independence, initially the Indian state continued with the British Indian arrangement dividing India into large provinces, also called ‘presidencies’. (Madras, Bombay, and Calcutta were the three major presidencies; incidentally, all three cities after which the presidencies were named have changed their names recently). These were large multi-ethnic and multilingual provincial states constituting the major political-administrative units of a semi-federal state called the Union of India. Soon after the adoption of the Constitution, all these units of the colonial era had to be reorganised into ethno-linguistic States within the Indian union in response to strong popular agitations."

This paragraph clearly shows that regionalism in India is closely connected to ethnic identities that are region-specific. Ethnicity provides a sense of shared belonging and identity within a geographical area, which has been the primary basis for demands for state reorganization and regional autonomy.

On the other hand, religion, though an important part of India’s cultural diversity, cuts across many regions and communities and does not correspond to specific geographic boundaries in the same way ethnicity does. Religious identities are more widespread and pan-Indian, while ethnicity is localized and directly linked to particular regions. Therefore, ethnicity rather than religion is the fundamental basis of regionalism in India.