The correct answer is: A) Iron oxides.
Iron oxides are the main reason for the color of black cotton soil. They are responsible for the red, yellow, brown, and black colors of soil. The amount of iron oxides in soil depends on the type of parent material, climate, and vegetation.
Organic matter is also a factor that affects the color of soil. However, it is not the main reason for the color of black cotton soil. Organic matter is responsible for the dark brown and black colors of soil.
Silica and clay particles are not the main reasons for the color of black cotton soil. Silica is the main component of sand, while clay particles are the main component of clay. Both silica and clay particles are colorless.
In conclusion, the color of black cotton soil is due to the presence of iron oxides.