Betel nut (areca nut) is a significant horticultural crop in Assam, primarily grown in:

The correct answer is: a) Nagaon and Kamrup districts.

Betel nut (areca nut) is a significant horticultural crop in Assam, primarily grown in the districts of Nagaon and Kamrup. These districts are located in the Brahmaputra Valley, which is a fertile region with a long history of agricultural production. Betel nut is a popular crop in Assam, and it is used in a variety of traditional and cultural practices.

The other options are incorrect for the following reasons:

• Option b) Barak Valley is a region in southern Assam that is not known for its betel nut production.
• Option c) Hill districts are located in the eastern part of Assam, and they are not as well-suited for betel nut cultivation as the Brahmaputra Valley.
• Option d) Upper Assam is a region in northern Assam that is known for its tea production, but it is not a major producer of betel nut.