An ecosystem consists of collection of biotic and abiotic components of the environment. Biotic factors are the biological factors. This means that they are living factors with which an organism can interact in an ecosystem. Such factors may include all living forms. Abiotic factors are non-biological components of an ecosystem. They are physical or non living things that influence the organism in some or the other way. Such factors may include rainfall, light, pH, temperature, etc.
Microorganisms are the biotic components. They are the living forms. Therefore, they do not comprise of the abiotic factors. Hence, option (a) is the correct option.
Abiotic factors include temperature, pH, rainfall, and light. Therefore, options (b), (c), (d), and (e) are incorrect.
Hence, the factor that is not abiotic among the given options is (a) microorganisms.