Abiotic factors are the nonliving aspecst of the enviroment. For example, the soil, the water, the temperature can be considered abiotic factors.
Biotic factors are the living aspects of the enviropent. For example animals or plants.
a. Mushrooms are living organisms, so they can be considered biotic factors.
b. Birds, so they are considered biotic factors.
c. Deers are living beings, they can be considered biotic factors.
d. The river is flowing water, water is an abiotic factor so the river can be considered an abiotic factor too.
The correct answer is d.