Question

why are the northern plains most densely populated regions of India

Answer 1

northern plains are densely populated because of better employments. As it is a plain and have fertile soil, there is much place for agriculture.So , people prefer to live in northern plains of India because of these factors.

Answer 2

good agricultural soil or you can say fertile land.the northern plains of course have plain topography the river ganga and yamuna and thier tributaries provide water and alluvial soil makes it a fertile land .thus it is highly populated