Black cotton soils are found in limited areas in Assam, primarily in:

The correct answer is: a) Karbi Anglong.

Black cotton soils are found in limited areas in Assam, primarily in the Karbi Anglong district. These soils are rich in organic matter and are very fertile. They are also known as regur soils or black earths. Black cotton soils are formed in areas with a hot, dry climate and a long dry season. They are well-drained and have a high water-holding capacity. These soils are ideal for growing crops such as cotton, rice, and sugarcane.

The other options are incorrect because:

• Nagaon is a district in Assam that does not have a significant amount of black cotton soil.
• Barak Valley is a region in Assam that does not have a significant amount of black cotton soil.
• Dhemaji is a district in Assam that does not have a significant amount of black cotton soil.