Step 1: Understanding the river.
The Kosi River, originating in Nepal, flows through north Bihar. Due to its frequent and devastating floods, it is known as the “Sorrow of Bihar.”
Step 2: Analyzing the options.
(A) Son: Not known as Sorrow of Bihar.
(B) Gandak: Causes floods but not called Sorrow of Bihar.
(C) Kosi: Correct — often changes course and causes destruction.
(D) Punpun: Smaller river, not associated with this title.
Step 3: Conclusion.
The “Sorrow of Bihar” is the Kosi River.