Question

why the northern plains of INDIA are one of the most densely populated regions of the world.

Answer 1

The northern plain of India gets enough rainfall for cultivation and the soil is prolific. The rivers of north India carries prolific soil for cultivation and also they are developed from high mountains region streams as they never get vanished. Northern India plains is also a floodplain that used grow crops. The foot region of mountains also produces as much as water energy resources too.
Hope it will help you.

Answer 2

northern plains of india are densely populated because plains are suitable with social economical and geographical changes.In geographical changes topography,climate,water availability ,soil,minerals etc.In social changes the northern plains are enriched with devotional activities and coming to economical northern plains are the occcupational areas which provide a livelihood for people