The correct answer is (c), West Bengal.
Assam is a state in north-eastern India. It is bordered by Bhutan to the north, Arunachal Pradesh to the north-east, Nagaland to the east, Manipur to the south-east, Mizoram to the south, Tripura to the south-west, Meghalaya to the west, and Bangladesh to the west and south.
Bihar is a state in eastern India. It is bordered by Nepal to the north, Uttar Pradesh to the west, Jharkhand to the south, West Bengal to the east, and the Bay of Bengal to the south-east.
Arunachal Pradesh is a state in north-eastern India. It is bordered by Bhutan to the north, China to the north-east, Nagaland to the east, Assam to the south, Meghalaya to the south-west, and Manipur to the west.
Nagaland is a state in north-eastern India. It is bordered by Assam to the north, Manipur to the south, and Myanmar to the east, west, and north-west.
West Bengal is a state in eastern India. It is bordered by Nepal to the north, Sikkim to the north-east, Bhutan to the east, Assam to the east and north-east, Meghalaya to the north-east, Jharkhand to the west, Odisha to the south-west, and Bangladesh to the south.
As you can see, West Bengal is the only state among the given options that does not share a border with Assam.