The correct answer is: C. organic colloids.
Humus is a dark, sticky substance that forms in soil when plant material decomposes. It is made up of complex organic molecules that are too small to be seen with the naked eye. Humus is an important part of soil because it helps to hold water and nutrients, and it also helps to improve soil structure.
Soil structure is the arrangement of soil particles into aggregates. Aggregates are groups of soil particles that stick together. Soil structure is important because it affects the ability of water and air to move through the soil, and it also affects the ability of plants to grow.
Crystalloids are solid particles that have a regular shape. They are formed when atoms or molecules arrange themselves in a repeating pattern. Crystalloids are found in many different materials, including rocks, minerals, and metals.
None of the above is not the correct answer because humus is an example of organic colloids.