Answer: d. all of the above
The soil microbes and insects plays an important role in maintaining the fertility of the soil. Also, the soil is the habitat of large diversity of these insects and microbes. The fungi, bacteria and insects- nematodes decomposes the plant litter in the form of leaves, twigs and other organic matter available in the soil and helps in preparation of humus in the soil. Humus is a dark colored, sticky substance which is rich in organic matter and supports the growth of the plants. The healthier the soil, the more plants will grow and more litter will produce which otherwise favor the soil ecosystem.