The correct answer is - a. soil color.
Even though every aspect of the soil changes when introduced continuously to a large amount of organic material, the most noticeable change will be the color of the soil. If the soil has very little organic material, than its color will be yellowish, grayish, reddish, but if the soil gets lots of organic material, than it will have dark brownish color, and even blackish, and that is due to the decomposition of the organic material.
