Answer:
Gran Chaco is a swamp because a swamp is an area of low-lying ground.
and Gran Chaco is a low-lying ground, with no mountains or rivers around it, and it is not on high ground.
why it is not any of the rest?
not a plateau because a plateau is an area of relatively level high ground.
not a mountain because a mountain is a mountain.
not a river because if you look at the map it's just grass, like a desert but it's not a desert.
