The correct answer is C. non-biodegradable chemicals.
Non-biodegradable chemicals are those that cannot be broken down by natural processes. They can remain in the environment for long periods of time, and can accumulate in the food chain, causing harm to animals and humans. Some examples of non-biodegradable chemicals include plastics, pesticides, and heavy metals.
Natural nutrients in excess can also be harmful to the environment, but they are not as harmful as non-biodegradable chemicals. Natural nutrients, such as nitrogen and phosphorus, are essential for plant growth. However, when they are present in excess, they can cause algae blooms, which can deplete oxygen in the water and kill fish and other aquatic life.
Human organic wastes, such as sewage and garbage, can also be harmful to the environment. However, they are not as harmful as non-biodegradable chemicals or natural nutrients in excess. Human organic wastes can be broken down by natural processes, and they do not accumulate in the food chain.
Waste animal feed is not a major source of environmental pollution. It is typically composted or incinerated, which destroys any harmful chemicals that may be present.
In conclusion, the most harmful environmental pollutants are non-biodegradable chemicals. They can remain in the environment for long periods of time, and can accumulate in the food chain, causing harm to animals and humans.