Q1. Which one of the following is the oldest oil refinery in India?
A Mathura refinery
B Koyali refinery
C Digboi refinery Correct
D Haldia refinery
Explanation
Digboi refinery in Assam is recognised as India's oldest oil refinery and is closely linked with the early history of petroleum production in the country. It began operations far earlier than the large public-sector refineries built after Independence. Mathura refinery, Koyali refinery, and Haldia refinery are important Indian refineries, but they were established much later. The question asks specifically for the oldest refinery, so the historically earliest and most appropriate answer is Digboi refinery.