Gandak river is associated with which one of the following river systems?

The correct answer is B, Ganga. The Gandak rises in the Nepal Himalaya, where it is called the Narayani or the Sapta Gandaki, enters India at Valmiki Nagar in West Champaran and flows south east across north Bihar to join the Ganga opposite Patna, near Sonpur and Hajipur. It is therefore a left bank tributary of the Ganga and belongs to the Ganga system, and the Gandak project barrage with its Tirhut and Saran canals is built on this river.

Option A, the Indus, drains the north west, with the Jhelum, Chenab, Ravi, Beas and Sutlej as its tributaries, none of them near Bihar. Option C, the Brahmaputra, flows through Assam far to the east and receives the Subansiri, Manas and Teesta. Option D, the Godavari, is a peninsular river flowing east into the Bay of Bengal through Maharashtra, Telangana and Andhra Pradesh. The north Bihar rivers, the Gandak, the Kosi, the Bagmati and the Burhi Gandak, are all left bank tributaries of the Ganga.