Detailed Explanation:

The Kosi River is called the 'Sorrow of Bihar' due to its frequent and devastating floods that destroy farmland and settlements. Originating from the Himalayas in Nepal, it carries massive sediment loads that cause the river to frequently change its course on the plains, making it highly unpredictable and destructive.

The river's braided channels and erratic behavior result in catastrophic flooding, turning fertile land into wasteland and causing immense human and economic loss in Bihar.

Therefore, Kosi is known as the Sorrow of Bihar, making Option 2 the correct answer.