FINAL ANSWER:
b. in relationship with other species.
Step-by-step explanation:
In natural circumstances, most microbes (microorganisms) typically exist in relationships with other species. Microbial communities can be highly diverse and complex, and these microorganisms often interact with each other and with other organisms in their environment. These interactions can include mutualistic relationships, where different species benefit from each other, competitive interactions, predation, and more.
Microbes can form symbiotic relationships with plants, animals, and other microbes. For example, some bacteria in the root nodules of leguminous plants fix nitrogen, benefiting both the plant and the bacteria. In aquatic ecosystems, various microorganisms are part of food webs and nutrient cycling, and in the human body, the microbiome consists of trillions of microbes that interact with our bodies in various ways.
So, option (b) is the most accurate representation of how microbes exist in natural circumstances, often in complex relationships with other species.