Ganga and Gandak is the answer that matches the Bihar-specific fact asked in the question. The Sonepur Mela (Harihar Kshetra Mela) is held at Sonepur in Saran district of Bihar, at the confluence of the Ganga and Gandak rivers. It is considered Asia's largest cattle fair and is held annually on Kartik Purnima (the full moon day of the Hindu month of Kartik, usually November). The fair has a history spanning thousands of years and is associated with the mythological story of Gajendra Moksha from the Bhagavata Purana, where Lord Vishnu rescued the elephant Gajendra from the grip of a crocodile at this very spot. During the fair, elephants, horses, cattle, and various animals are traditionally traded. The Ganga and Son meet near Patna (Arrah area), not at Sonepur. The Ganga and Kosi confluence is at Kursela in Katihar district. The Ganga and Bagmati confluence is in the Muzaffarpur-Vaishali area. The Sonepur fair also has significant cultural, commercial, and religious importance and attracts visitors from across India and abroad. Bihar GK questions frequently place neighbouring districts, rivers, historical centres, personalities or government institutions together as distractors, so recognition of a familiar name alone is insufficient. The alternatives Ganga and Son, Ganga and Kosi, Ganga and Bagmati may be valid Bihar-related terms, but they do not satisfy the complete relationship asked here. A sound method is to identify the subject area first—geography, history, polity, economy or culture—and then verify the location, period and function separately.