The correct answer is option 3: Digboi.

Haldia, option 1, is not the oldest oil refinery in India. Koyali, option 2, is not the oldest either. Mathura, option 4, is also not the oldest oil refinery in India.

Digboi, option 3, is the oldest oil refinery in India. It was established in 1901 and is located in the Tinsukia district of Assam. The refinery was set up by Assam Oil Company Limited (a subsidiary of Indian Oil Corporation Limited) and is known for its historical significance in the Indian petroleum industry.

Digboi Refinery played a crucial role in the development of the Indian oil industry and has a remarkable heritage. It produced the country`s first petroleum product and has continued to be operational for more than a century. The refinery contributes significantly to the economy of the region and continues to play a pivotal role in meeting India`s energy needs.

Therefore, the oldest oil refinery in India is Digboi, not Haldia, Koyali, or Mathura.