The quality of the soil is decided by:

The correct option is C

both (a) and (b)

The correct option is C.

Explanation of the correct options:

1. Microorganisms present in the soil are responsible for the the breakdown of the organic substances.
2. The organisms found in the soil are bacteria, actinomycetes, fungi, protozoa, and nematodes.
3. Plants shed their leaves and other products which are being eaten by these organisms and they convert them into humus.
4. Humus is the substance which is very rich in nutrient and black in color.
5. Humus provides nutrients to the plants and also provide good and healthy growth to the plants.

Final answer: The quality of the soil is decided by the amount of humus and microscopic organisms.