Explanation:

• The river Kosi is formed by the confluence of three streams namely the Sun Kosi, the Arun Kosi and Tamur Kosi all taking their origin in the Himalayan region of Nepal and Tibet.
• Apart from these three streams which form the Kosi, a number of tributaries join the river in course of its flows through the plains from the right bank. These are Trijunga, Bhutahi Balan, Kamla Balan and Bagmati.

Hence option 2nd is correct.