The correct answer is: b) The Office of the Registrar General & Census Commissioner.
The Office of the Registrar General & Census Commissioner is a government agency under the Ministry of Home Affairs of the Government of India. It is responsible for conducting the decennial census of India. The census is a comprehensive survey of the population of India, and it provides data on a variety of topics, including age, sex, religion, language, and literacy. The census is used to plan for the development of India, and it is also used to allocate resources to different parts of the country.
The other options are incorrect. The State Government of Sikkim is a state government in India. International NGOs are non-governmental organizations that operate internationally. Private research firms are companies that conduct research for a variety of clients.