Hello there!
Your answer would be "Ganges"
When you plug your answer in the question, it'll say:
The Ganges plain stretches across northern India and is home to one-tenth of the world's population.
The reason why "Ganges" would be the correct answer is because this plain stretches all the way from the Ganges River and the Brahmaputra River Valley all the way to the Indus River Valley. This would be a total of 2.55 million square kilometers, or 2,550,000,000,000 square meters. That is a lot of land, and places that have a lot of land could hold up to a highly dense population. And this is where 1/10 of the world's population lives, and 1/10 of the world's population is 750,000,000 people, that is A LOT of people in one land, and India isn't that big at all compared to other places. This is the reason why this would be your answer.