The correct option is Dhubri.
Explanation
The Brahmaputra river, after originating in the Chemayungdung glacier and traversing Tibet as the Yarlung Tsangpo, enters India through Arunachal Pradesh. It then flows southwest through the Assam Valley as a braided river.
Geographical Analysis:
- The river flows longitudinally through the Assam Valley from east to west.
- Upon reaching the western extremity of the state of Assam, near Dhubri, the river takes a sharp southward turn (approximately 90°) around the Garo Hills.
- This turn marks the river's exit from India and its entry into Bangladesh.
- Once it enters Bangladesh, the river is known as the Jamuna, where it eventually joins the Padma (Ganga) and the Meghna system.
Key Takeaway: Dhubri is the specific location in Assam where the Brahmaputra turns south to enter Bangladesh.