The river referred to as the Tsang-po in Tibet is the Brahmaputra. It is also known by this name in the region. Option 1, Ganga, is incorrect because the Ganga is a different river altogether and is not known as the Tsang-po in Tibet. Option 3, Indus, is also incorrect because the Indus is a separate river and is not referred to by the name Tsang-po. Option 4, Teesta, is also incorrect because the Teesta is a different river and not known as the Tsang-po in Tibet. Therefore, the correct answer is option 2, Brahmaputra.