From which year, the census became a regular ten-yearly exercise conducted by the British Indian government?

1881

The correct answer is Option 2: 1881

NCERT : But by far the most important official effort to collect information on caste was through the census. First begun in the 1860s, the census became a regular ten-yearly exercise conducted by the British Indian government from 1881 onwards. The 1901 Census under the direction of Herbert Risley was particularly important as it sought to collect information on the social hierarchy of caste – i.e., the social order of precedence in particular regions, as to the position of each caste in the rank order. This effort had a huge impact on social perceptions of caste and hundreds of petitions were addressed to the Census Commissioner by representatives of different castes claiming a higher position in the social scale and offering historical and scriptural evidence for their claims. Overall, scholars feel that this kind of direct attempt to count caste and to officially record caste status changed the institution itself. Before this kind of intervention, caste identities had been much more fluid and less rigid; once they began to be counted and recorded, caste began to take on a new life