The correct answer is: a) Assam.
Assam is a state in north-eastern India. It is bordered by Bhutan to the north, China’s Tibet Autonomous Region 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 West Bengal to the west and northwest. The Brahmaputra River flows through the state, and the capital is Dispur.
Meghalaya is a state in north-eastern India. It is bordered by Assam to the north, Nagaland to the east, Manipur to the south, and Bangladesh to the west. The capital is Shillong.
Nagaland is a state in north-eastern India. It is bordered by Assam to the north, Arunachal Pradesh to the east, Manipur to the south, and Myanmar to the west. The capital is Kohima.
Tripura is a state in north-eastern India. It is bordered by Bangladesh to the north, east, and west, Mizoram to the south-west, and Assam to the south-east. The capital is Agartala.
Assam is the only state that lies to the south of Arunachal Pradesh.