The correct answer is option 3 - Digboi. Digboi, located in the state of Assam, was the site of the first oil refinery in India. It was established in the year 1901 by Assam Oil Company, making it the oldest refinery in the country. Digboi played a significant role in the development of India`s petroleum industry and is often referred to as the "Oil City of Assam." The refinery at Digboi was initially set up to process the crude oil extracted from the nearby oil fields and has played a crucial role in meeting the country`s energy demands. It continues to be operational to this day and is considered a historic landmark in the Indian petroleum industry.