The correct answer is river Mahananda.

• Mahananda river is an important tributary of Ganga river.
• It flows in the region of northern India and Bangladesh.
• It rises in the Darjiling hills in extreme northern West Bengal state. 
• The river flows south through a rich agricultural area in Bihar state, enters West Bengal state, and then in Bangladesh to join the Ganga river.

• Brahmputra River is 2,900 km in length.
• Tributaries of Brahmaputra River are some of the major rivers of North-east India:
  • They are Lohit River, Dibang River, Subansiri River,Dhansiri River, Manas River, Torsa River, Dihing River, Teesta River, Sankosh River, Kopili River, Raidak River and Bhareli River. 
  • Manas River:
    • It is a transboundary river in the Himalayan foothills between southern Bhutan and India.
    • It is the largest river system in Bhutan.
    • The total length of the river is 376 kilometers.
  • Kameng River:
    • It flows in Arunachal Pradesh and Jia Bhoreli in Assam and earlier it was known as Bhareli River.
    • It is about 264 kilometers long. 
  • Subansiri River:
    • It is also known as the "Gold River" and a large tributary of the Brahmaputra River in Arunachal Pradesh.
    • It flows through Assam, Arunachal Pradesh, and the Tibet Autonomous Region of China.
    • The length of the Subansiri River is 442 km long.