Regionalism in India can be understood with reference to:

Ethnicity

The correct answer is Option (3) → Ethnicity

Regionalism in India primarily refers to the expression of identity, interests, and concerns based on geographical regions within the country. Therefore, it is best understood with reference to ethnicity, as it often involves a sense of belonging and attachment to a particular region, along with the desire for regional autonomy or greater recognition of regional issues. While caste, class, and religion may also intersect with regionalism, ethnicity is the most directly relevant factor in this context.