The correct answer is (a) Upper Assam.
Upper Assam is a region in the north-eastern Indian state of Assam. It is the largest and most populous region in Assam, accounting for about 60% of the state’s population. The region is known for its rich oil and natural gas reserves, which are primarily found in the Brahmaputra Valley. The oil and natural gas reserves in Upper Assam are estimated to be among the largest in India.
The other options are incorrect.
(b) Lower Assam is a region in the south-eastern Indian state of Assam. It is the second largest and second most populous region in Assam, accounting for about 30% of the state’s population. The region is known for its fertile land and its production of rice and tea.
(c) Barak Valley is a region in the south-western Indian state of Assam. It is the third largest and third most populous region in Assam, accounting for about 10% of the state’s population. The region is known for its hilly terrain and its production of tea and rubber.
(d) Karbi Anglong is a district in the north-eastern Indian state of Assam. It is the largest district in Assam, accounting for about 15% of the state’s area. The district is known for its hilly terrain and its production of coal and timber.