The top part of which of the following hill/range/doon is known as the boundary line between Bihar

Q. The top part of which of the following hill/range/doon is known as the boundary line between Bihar and Nepal?
Answer: Someshwara Range
Notes:

The top part of the Someshwar range is known as the boundary line between Bihar and Nepal. This range is 74 km. is long. The range lies in the West Champaran district of Bihar.