The correct answer is (b) River plains.
The river plains of Assam are the most fertile region of the state. The Brahmaputra River and its tributaries have deposited rich alluvial soil in the plains, making it ideal for agriculture. The climate in the river plains is also favorable for agriculture, with warm temperatures and abundant rainfall. As a result, the river plains of Assam have a higher cropping intensity than other regions of the state.
Hilly areas (a) are less fertile than the river plains and have a shorter growing season. Karbi Anglong (c) is a hilly district in Assam with a population of over 1 million people. Barak Valley (d) is a region in Assam with a population of over 2 million people. Both Karbi Anglong and Barak Valley have a lower cropping intensity than the river plains.