Which long term process was most decisive in the formation of the Northern Plains?

Correct answer: Alluvial deposition by Himalayan rivers

Direct answer: Option A, alluvial deposition by Himalayan rivers. The Northern Plains developed in a large low-lying basin south of the Himalaya. Rivers such as the Indus system, Ganga system, and Brahmaputra carry sediments eroded from mountains and deposit them over very long periods. Repeated deposition builds thick, extensive layers of alluvium, creating flat and fertile plains. Option A is correct because river-borne alluvium is the decisive long-term process. Option B is wrong because lava flows form volcanic igneous surfaces, not the broad alluvial Northern Plains. Option C is wrong because marine coral accumulation is limited to suitable shallow warm seas and is not the main origin of these plains. Option D is wrong because wind-blown desert sand is local and cannot explain the enormous river-built plain. Memory cue: Himalayan rivers bring sediment; sediment builds plains.