Which river forms the southern boundary of Assam with Mizoram?

The correct answer is: a) Barak

The Barak River is a major river in the northeastern Indian states of Assam and Mizoram. It is the largest tributary of the Brahmaputra River. The Barak River originates in the Manipur Hills and flows through the Barak Valley in Assam before joining the Brahmaputra River near Karimganj. The Barak River is a major source of irrigation and drinking water for the people of Assam and Mizoram. It is also a popular destination for fishing and boating.

The other options are incorrect because:

• The Dhansiri River is a tributary of the Brahmaputra River. It flows through the Naga Hills and the Karbi Anglong district of Assam.
• The Dihing River is a tributary of the Brahmaputra River. It flows through the Dibrugarh district of Assam.
• The Manas River is a tributary of the Brahmaputra River. It flows through the Manas National Park in Assam.