When someone says that he/she is a Hindu or a Muslim or an Adivasi or a North Indian or South Indian, what is he/she trying to refer to?

Social identity

The correct answer is Option (1) → Social identity

Social identity refers to those aspects of a person that link her/him to a social or cultural group or are derived from it. When someone says that s/he is a Hindu or a Muslim, a Brahmin or an adivasi or a North Indian or a South Indian, or something like these, s/he is trying to indicate her/his social identity. These descriptions characterise the way people mentally represent themselves as a person.