The correct answer is: Dhubri.
Dhubri district shares the longest international boundary with Bangladesh. The total length of the international boundary between Dhubri district and Bangladesh is 221.6 km. The boundary starts from the confluence of the Brahmaputra and the Manas rivers in the north and runs south to the confluence of the Brahmaputra and the Hariabhanga rivers in the south.
Karimganj district shares an international boundary with Bangladesh in the south. The total length of the international boundary between Karimganj district and Bangladesh is 140.4 km. The boundary starts from the confluence of the Brahmaputra and the Kushiyara rivers in the north and runs south to the confluence of the Brahmaputra and the Hariabhanga rivers in the south.
Kokrajhar district shares an international boundary with Bhutan in the north. The total length of the international boundary between Kokrajhar district and Bhutan is 110.4 km. The boundary starts from the confluence of the Brahmaputra and the Manas rivers in the north and runs south to the confluence of the Brahmaputra and the Sankosh rivers in the south.
Silchar district does not share any international boundary.