Option (A) Assam is correct because the textbook explicitly states that 'In Assam, Digboi, Naharkatiya and Moran are important oil producing areas.' Additionally, it mentions that petroleum reserves are located in the sedimentary basins of Assam, Gujarat and Mumbai High. The context also notes that 'Till then, Digboi in Assam was the only oil producing region' before 1956, establishing Assam's historical significance in oil production. Options (B) Bihar, (C) Rajasthan, and (D) Tamil Nadu are not mentioned as having major oil fields in the provided context.