Tribes are classified into four categories on the basis of which of the following?

Language

The correct answer is Option 2: Language

NCERT: "In terms of language, tribes are categorised into four categories. Two of them, Indo-Aryan and Dravidian, are shared by the rest of the Indian population as well, and tribes account for only about 1% of the former and about 3% of the latter. The other two language groups, the Austric and Tibeto-Burman, are primarily spoken by tribals, who account for all of the first and over 80% of the second group. In physical-racial terms, tribes are classified under the Negrito, Australoid, Mongoloid, Dravidian and Aryan categories. The last two are again shared with the rest of the population of India."