Final answer:
Complex communities of microorganisms that form on various environmental surfaces are termed biofilms, which are highly structured and offer a selective advantage to their constituent microbes. Therefore, the correct option is C.
Step-by-step explanation:
Complex communities of microorganisms on surfaces are called biofilms. These are not random collections of microorganisms but are highly structured communities that offer a selective advantage to their constituent microorganisms. In nature, microorganisms mostly exist in biofilms, which tend to form on various surfaces exposed to a liquid environment with some nutrients. These can include surfaces like industrial pipelines, riverbed rocks, or even within the human body such as the mouth where biofilms may contain hundreds of bacterial species. Biofilms are made up of a matrix primarily composed of polysaccharides, proteins, and nucleic acids secreted by the microorganisms within them.