Option-1: River Manas - This is incorrect. River Manas is indeed a tributary of river Brahmaputra. It originates in Bhutan and joins Brahmaputra in Assam, India.

Option-2: River Kameng - This is incorrect. River Kameng is also a tributary of river Brahmaputra. It flows through Arunachal Pradesh in India and joins Brahmaputra in Assam.

Option-3: River Mahananda - This is the correct answer. River Mahananda is not a tributary of river Brahmaputra. It flows through the states of Bihar and West Bengal in India and eventually joins the Ganges river.

Option-4: River Subansiri - This is incorrect. River Subansiri is indeed a tributary of river Brahmaputra. It originates in Tibet and flows through Arunachal Pradesh before joining Brahmaputra in Assam.

Therefore, the correct answer is option 3 - River Mahananda. It is not a tributary of river Brahmaputra as it joins the Ganges river instead.