The Tsangpo river is also known as the Brahmaputra river in Tibet. The Tsangpo river originates in western Tibet and flows through the Indian state of Arunachal Pradesh before merging with the Ganga river in Bangladesh. Option 1, Kosi, is incorrect as the Kosi river is a tributary of the Ganga, not the Tsangpo. Option 2, Gandak, is also incorrect as the Gandak river is a tributary of the Ganga, not the Tsangpo. Option 4, Ganga, is incorrect because although the Tsangpo eventually merges with the Ganga, they are distinct rivers. Therefore, the correct answer is option 3, Brahmaputra.