The correct answer is: a) The oldest oilfield in India.
The Digboi oilfield is located in the northeastern Indian state of Assam. It was discovered in 1889 and is the oldest oilfield in India. The oilfield is operated by Oil India Limited, a state-owned oil and gas company. The Digboi oilfield has produced over 1 billion barrels of oil since its discovery.
The other options are incorrect. Option b) is incorrect because the largest onshore oilfield in India is the Mangala oilfield, which is located in the western Indian state of Rajasthan. Option c) is incorrect because the deepest oil well in India is the KG-D6 well, which is located in the Bay of Bengal. Option d) is incorrect because the highest quality crude oil producer in India is the Mumbai High oilfield, which is located in the Arabian Sea.