The dark-colored soil is richest in organic matter. Decaying organic matter, especially plant matter, turns brownish-black - look at potting soil, which is rich in organic material and is almost black.
Yellow soil may contain oxides of iron (inorganic) to give it that color, and blue or red soil may contain clay, which is powdered feldspar or feldspathoid minerals, which are also artificial. So your best answer choice is B.
