The correct answer is: d) Brahmaputra Valley
The Brahmaputra Valley is the largest river valley in Assam. It is a fertile plain that is well-suited for agriculture. The valley is also home to a number of important rivers, including the Brahmaputra, the Barak, and the Dibang. These rivers provide water for irrigation and drinking, as well as transportation.
The Brahmaputra Valley is also home to a number of important industries, including tea, oil, and natural gas. The valley is a major economic hub for Assam and India.
The other options are incorrect because they are not as large or as important as the Brahmaputra Valley. The Barak Valley is a smaller river valley that is located in the south of Assam. The Kopili Valley is a smaller river valley that is located in the west of Assam. The Dhansiri Valley is a smaller river valley that is located in the north of Assam.
The Brahmaputra Valley is the most important river valley in Assam because it is the largest, most fertile, and most economically important.