The correct answer is - flood plain.
The flood plains are flat areas, they are mostly elongated in shape, and cover the space around the rivers, especially around the big rivers. When there's a big amount of water coming into the river in a short period of time, be it from heavy rainfall, or from melting snows, the water level of the rivers rises and floods these plains. With every flooding, the rivers leave lots of deposits on the flood plains, and that results in high quality fertile soil which is excellent for farming.