The correct option is
E Soil
The environment is a complex of many factors which surround an organism. These factors are known as environmental factors or ecological factors which may be either living or biotic and non-living or biotic.
Biotic factors include plants, animals, microbes, etc.
Abiotic factors are the non-living factors, substances, and conditions of the environment which influence the physiology, survival, function, and reproduction of organisms.
Hence, trees, mushrooms, squirrels, and bacteria are biotic factors while the soil is an abiotic factor.
So, the correct answer is 'Soil'.