The correct answer is: a) Digboi.
Digboi is a town in Tinsukia district in the Indian state of Assam. It is located on the banks of the Brahmaputra River, about 500 kilometers (310 mi) east of Guwahati. Digboi is known for its oil refinery, which is the oldest in Asia. The refinery was established in 1889 by the Assam Oil Company. It is a major producer of petroleum products, including gasoline, diesel, and kerosene.
The other options are incorrect because they are not the locations of oil refineries. Bongaigaon is a town in Bongaigaon district in the Indian state of Assam. It is located on the banks of the Brahmaputra River, about 200 kilometers (120 mi) west of Guwahati. Bongaigaon is a major railway junction and a commercial center. Guwahati is the largest city in Assam and the capital of the state. It is located on the banks of the Brahmaputra River, about 200 kilometers (120 mi) east of Digboi. Guwahati is a major commercial and educational center. Numaligarh is a town in Golaghat district in the Indian state of Assam. It is located on the banks of the Brahmaputra River, about 200 kilometers (120 mi) east of Guwahati. Numaligarh is a major industrial center.