The major oil refineries in Assam are located in:

Digboi and Guwahati
Bongaigaon and Namrup
Jorhat and Tinsukia
Silchar and Dibrugarh

The correct answer is: b) Bongaigaon and Namrup.

Digboi is an oil town in Tinsukia district of Assam, India. It is the oldest oil field in Asia and the first commercial oil well in India was drilled here in 1889. The town is also home to the Digboi Refinery, which is one of the oldest refineries in India.

Guwahati is the largest city in Assam and the capital of the state. It is a major commercial and educational center in the northeast region of India. The city is home to a number of oil refineries, including the Numaligarh Refinery and the Bongaigaon Refinery and Petrochemicals Limited.

Bongaigaon is a town in Bongaigaon district of Assam, India. It is located on the banks of the Brahmaputra River. The town is home to the Bongaigaon Refinery and Petrochemicals Limited, which is one of the largest refineries in India.

Namrup is a town in Dibrugarh district of Assam, India. It is located on the banks of the Brahmaputra River. The town is home to the Namrup Refinery, which is one of the oldest refineries in India.

Jorhat is a city in Jorhat district of Assam, India. It is the second largest city in Assam and the headquarters of the Jorhat district. The city is home to a number of oil refineries, including the Numaligarh Refinery and the Bongaigaon Refinery and Petrochemicals Limited.

Tinsukia is a town in Tinsukia district of Assam, India. It is located on the banks of the Brahmaputra River. The town is home to a number of oil refineries, including the Digboi Refinery and the Numaligarh Refinery.

Silchar is a city in Cachar district of Assam, India. It is the largest city in the Barak Valley region of Assam. The city is home to a number of oil refineries, including the Numaligarh Refinery.

Dibrugarh is a city in Dibrugarh district of Assam, India. It is the third largest city in Assam and the headquarters of the Dibrugarh district. The city is home to a number of oil refineries, including the Numaligarh Refinery and the Bongaigaon Refinery and Petrochemicals Limited.