Answer: The correct answer for the blank is- b. rich in minerals and organic material.
Topsoil is the outermost layer of the soil, which is generally 5 to 10 inches in thickness. This layer is enriched with minerals, organic matter, and microorganisms. It is the site of maximum biological activity that occur on the earth. Roots of plants are also concentrated in this layer so that they can absorb all the nutrients essential for the growth of plant.
Thus, option B) rich in minerals and organic material is the right answer.