Decomposer is the floorboard of all heterotrophs that can decompose complex organic components in animal and plant residues into simple compounds that can be reused by producers. Generally speaking, it refers to bacteria and fungi living in saprophytes, while bacteria and viruses living in parasitic organisms absorb organic matter from living animals and plants and do not belong to decomposers. Autotrophic microorganisms do not belong to decomposers.

So choose: D.