The correct answer is: b) River plains
The river plains of Assam are the most fertile region in the state. The soil is rich in nutrients and the climate is ideal for agriculture. The river plains are also well-irrigated, which makes it possible to grow two or three crops a year.
The hilly areas of Assam are less fertile and the climate is not as ideal for agriculture. The hilly areas are also not as well-irrigated, which makes it possible to grow only one crop a year.
Karbi Anglong is a district in Assam that is located in the hilly areas. The district is not as fertile as the river plains and the climate is not as ideal for agriculture. Karbi Anglong is also not as well-irrigated, which makes it possible to grow only one crop a year.
Barak Valley is a region in Assam that is located in the Barak River valley. The region is not as fertile as the river plains and the climate is not as ideal for agriculture. Barak Valley is also not as well-irrigated, which makes it possible to grow only one crop a year.