Step 1: Understanding Bihar’s geography.
Northern Bihar lies in the plains of the Ganga and its tributaries, especially the Kosi, Gandak, and Bagmati rivers. These rivers frequently flood the region.
Step 2: Analyzing the options.
(A) Eastern: Eastern Bihar does face floods but not as severely as the north.
(B) Northern: Correct — this region is most flood-prone due to Himalayan rivers.
(C) Southern: Comparatively less affected.
(D) Western: Also not as vulnerable as the northern region.
Step 3: Conclusion.
The part of Bihar most prone to floods is Northern Bihar.