The correct answer is: a) Nagaon.
Nagaon is the most populous district in Assam, with a population of 3,338,742 as of 2011. It is located in the central part of the state and is bordered by the districts of Golaghat, Karbi Anglong, Dima Hasao, Cachar, and Sonitpur. The district headquarters is at Nagaon town.
Nagaon is a major agricultural district and is known for its production of rice, tea, and jute. The district is also home to a number of industries, including tea processing, oil refining, and cement manufacturing.
Nagaon is a popular tourist destination and is home to a number of historical and religious sites, including the Kamakhya Temple, the Umananda Temple, and the Rang Ghar.
The other options are incorrect because:
- Kamrup Metropolitan is the second most populous district in Assam, with a population of 2,908,146 as of 2011.
- Barpeta is the third most populous district in Assam, with a population of 2,457,511 as of 2011.
- Dhubri is the fourth most populous district in Assam, with a population of 1,850,855 as of 2011.