The correct answer is (a) Upper Assam.
Upper Assam is the region of Assam that is located in the north of the state. It is the most populous region of Assam, with a population of over 20 million people. The region is known for its oil fields, which are located in the Brahmaputra Valley. The oil fields in Upper Assam are the largest in India, and they account for a significant portion of the country’s oil production.
The other options are incorrect.
(b) Barak Valley is a region of Assam that is located in the south of the state. It is a hilly region, and it is home to a number of tea gardens. The region is not known for its oil fields.
(c) Lower Assam is a region of Assam that is located in the south of the state. It is a lowland region, and it is home to a number of rivers, including the Brahmaputra River. The region is not known for its oil fields.
(d) Karbi Anglong is a district of Assam that is located in the north of the state. It is a hilly region, and it is home to a number of tribal people. The region is not known for its oil fields.