Concept:The Kosi River is called the 'Sorrow of Bihar' due to its frequent, devastating floods that ruin farmland.
Explanation:The Kosi has seven streams (Sut Kosi, Tamba Kosi, Talkha, Doodh Kosi, Botia Kosi, Arun, Tamber).
These streams come from snowy, high-rainfall areas in eastern Nepal (Sapt Kaushik region).
They merge into three main streams (Tumar, Arun, Sun Kosi) and unite at Triveni to form the Kosi.
The river cuts a gorge through the Mahabharata Range and enters Nepal's Tarai, then flows into Bihar.
Once it reaches the plains, its flow slows and it deposits huge amounts of sediment.
This makes the channel braided and prone to shifting course, causing severe floods.
The floods turn fertile land into wasteland, earning the Kosi the title 'Sorrow of Bihar'.
It joins the Ganga near Kursela in Katihar district.
Answer:B. Kosi