What is the main factor that makes the Northern Plains fertile?

Correct answer: New alluvium brought by rivers

The direct answer is B: new alluvium brought by rivers. The Northern Plains were built mainly by the deposition of sediments carried by the Indus, Ganga and Brahmaputra river systems and their tributaries. Rivers bring sand, silt and clay from higher areas and spread these materials across the plains, especially during floods. Step by step: weathering produces rock particles; rivers transport them; the flow becomes slower on the plain; sediments are deposited; repeated deposition creates deep, fertile alluvial soil. Option B is correct because fresh alluvium continually adds mineral material and helps maintain fertility. Option A, black lava soil, is mainly associated with basaltic volcanic regions such as parts of the Deccan Plateau, not the main origin of the Northern Plains. Option C, dry sandy soil, generally has low moisture retention and is not the chief reason for fertility. Option D, rocky mountain soil, is not the thick deposited soil of the plains. Fertility may also depend on irrigation and cultivation, but the main geographical factor asked here is river-borne alluvium. Memory cue: Northern Plains = rivers plus alluvium plus fertile fields.