The correct answer is: B) Plateaus.
Red soils are generally found in plateaus because they are formed from the weathering of igneous rocks, which are rich in iron oxide. The iron oxide gives the soil its red color. Plateaus are also typically dry and have low rainfall, which helps to preserve the red color of the soil.
Option A: River valleys are typically low-lying areas that are prone to flooding. This flooding can wash away the red soil, making it less likely to be found in these areas.
Option C: Hilly regions are also prone to erosion, which can wash away the red soil. Additionally, the higher elevation of hilly regions means that they are typically cooler and wetter than plateaus, which can also contribute to the loss of red soil.
Option D: Coastal areas are typically sandy or muddy, and they are not typically associated with red soil.