The correct answer is: b) Hilly regions.
Contour farming is a technique used to prevent soil erosion in hilly regions. It involves plowing and planting crops across the slope of the land, rather than up and down the slope. This helps to keep the soil in place and prevents it from being washed away by rain or wind.
Terracing is another technique used to prevent soil erosion in hilly regions. It involves creating a series of level steps on the side of a hill. These steps help to slow down the flow of water and prevent it from eroding the soil.
Option a) Coastal areas are not suitable for contour farming or terracing because the soil is too sandy and loose. Option c) River valleys are also not suitable for these techniques because the soil is too wet and prone to flooding. Option d) Flat plains are not as susceptible to soil erosion as hilly regions, so contour farming and terracing are not as necessary in these areas.