The river Brahmaputra is known as "Dihang" as it enters Arunachal Pradesh. It is important to note that the river changes its name as it flows through different regions. When it enters Arunachal Pradesh, it is referred to as Dihang. However, it is important to be aware of the other options as well to avoid confusion. Option 1, Dibang, is not the correct answer in this case. Option 3, Subansiri, and option 4, Dhansiri, are also not the names of the river Brahmaputra as it enters Arunachal Pradesh. Therefore, the correct answer is option 2, Dihang.