Which river forms a significant part of the southern boundary of Arunachal Pradesh?

The correct answer is: Brahmaputra.

The Brahmaputra is the longest river in India and the third longest river in Asia. It flows through the states of Arunachal Pradesh, Assam, Meghalaya, and Bangladesh. The Brahmaputra forms a significant part of the southern boundary of Arunachal Pradesh.

The Brahmaputra is a major source of water for irrigation, drinking water, and hydroelectric power. It is also an important transportation route. The Brahmaputra is a major river in the Indian subcontinent and is an important part of the culture and economy of the region.

The other options are incorrect because:

• The Ganga is a river in northern India and Bangladesh. It is the longest river in India and the second longest river in Asia. The Ganga does not form a significant part of the southern boundary of Arunachal Pradesh.
• The Teesta is a river in northeastern India and Bangladesh. It is the third longest river in Bangladesh. The Teesta does not form a significant part of the southern boundary of Arunachal Pradesh.
• The Barak is a river in northeastern India and Bangladesh. It is the longest river in Manipur. The Barak does not form a significant part of the southern boundary of Arunachal Pradesh.