Correct Answer: (b) Kosi River
Solution:The river Kosi, also called Koshi, is called the sorrow of Bihar. The Koshi stretches around 720 km in length and drains an area of about 74,500 kms across Tibet, Nepal and Bihar. The river derived its name as the sorrow of Bihar due to the annual floods that afflict the state most of the time during the rainy season. The flood such merge the agricultural land and destroys the rural economy. About 21,000 km² area gets drowned in floods caused by this river.