The correct answer is: a) Iron oxides.
Iron oxides are the main cause of the red color of red soils. They are formed when iron-rich minerals are exposed to oxygen and water. The amount of iron oxide in a soil can vary depending on the climate, the type of parent material, and the amount of weathering that has occurred.
Aluminum oxides are also found in red soils, but they do not contribute to the red color. They are white or gray in color.
Organic matter is found in all soils, but it is not usually present in large enough amounts to affect the color of the soil. It is usually brown or black in color.
Clay minerals are also found in all soils, but they do not usually affect the color of the soil. They are usually white or gray in color.
In addition to iron oxides, red soils may also contain other minerals that contribute to their color, such as manganese oxides and clay minerals. The amount of these minerals can vary depending on the location of the soil.