The fair is the Sonepur Mela — option 2.

Where and when. Sonepur stands in Saran district of Bihar, at the point where the Gandak joins the Ganga, opposite Hajipur and a little downstream of Patna. The fair opens on Kartik Purnima, the full moon of the month of Kartik in November or December, and runs for about a month — sometimes longer.

What it is. Sonepur is famous above all as a livestock fair — cattle, buffaloes, horses, and formerly elephants in large numbers, though the trade in elephants is now prohibited under the Wild Life (Protection) Act. Alongside the animal market run stalls of handicrafts, handloom, farm implements and food, and traditional entertainments. Its origins are ancient: pilgrims bathe at the confluence, and the fair is associated with the temple of Harihar Nath, said to mark the spot of the Gaj-Grah legend from the Bhagavata Purana, in which Vishnu rescues the elephant from the crocodile.

The historical trade. Chandragupta Maurya is traditionally said to have bought elephants and horses here, and the fair drew merchants from across northern India and Central Asia; the East India Company later purchased horses at Sonepur for its cavalry.

Why option 4 fails : only one of the named fairs is at the Ganga-Gandak confluence, so “more than one of the above” cannot apply. Katihar and Purnia are districts in the Seemanchal region, far to the east.

Hence, the answer is Sonepur Mela.