The correct answer is option 4, Digboi. Digboi oilfield is situated in the North-eastern region of India.
Let`s dissect the other options to understand why they are not the correct answers:
1) Kalol: Kalol oilfield is located in the state of Gujarat, which is in the western region of India, not in the North-east.
2) Ankaleshwar: Ankaleshwar oilfield is also situated in Gujarat, not in the North-eastern region of India.
3) Mehsana: Mehsana oilfield is located in the state of Gujarat as well, and not in the North-eastern region.
Therefore, the correct answer is option 4, Digboi, as it is the only oilfield in the given options that is situated in the North-eastern region of India.