The correct answer is: a) River valleys.
Soils with a coarse texture and good drainage are found in river valleys because they are constantly being eroded by the river. This erosion exposes the underlying rock, which is often coarse-grained and well-drained. The river also deposits sediment in the valley, which helps to create a fertile soil.
The Terai region is a flat, marshy plain in Nepal. The soil in the Terai is mostly clay and silt, which makes it poorly drained. Hill slopes are also poorly drained, as water tends to run off the surface of the slope. Flood plains are periodically inundated by floodwaters, which can deposit sediment and nutrients, but they can also erode the soil.
In conclusion, soils with a coarse texture and good drainage are found in river valleys because they are constantly being eroded by the river. This erosion exposes the underlying rock, which is often coarse-grained and well-drained. The river also deposits sediment in the valley, which helps to create a fertile soil.