The correct answer is: Iron oxides.
Iron oxides are the primary cause of the red color of red soils. They are formed when iron-rich minerals, such as hematite and magnetite, are exposed to oxygen and water. The iron oxides react with these elements to form new minerals that have a red or reddish-brown color.
Calcium carbonates are another common mineral found in soils. They are white or colorless, and they do not contribute to the red color of red soils.
Organic matter is a complex mixture of materials that are derived from living organisms. It is usually dark brown or black in color, and it does not contribute to the red color of red soils.
Silica is a common mineral that is found in many different types of rocks and soils. It is colorless or white, and it does not contribute to the red color of red soils.
In addition to iron oxides, red soils may also contain other minerals that contribute to their color, such as aluminum oxides and manganese oxides. The color of a soil can also be affected by the amount of organic matter it contains, as well as the pH of the soil.