An organism that can exist in both oxygen and oxygen-free environments is a?

The correct answer is that an organism capable of existing in both oxygen and oxygen-free environments is classified as a facultative anaerobe. This is because facultative anaerobes are versatile organisms that can utilize oxygen for aerobic respiration when it is available but can also switch to anaerobic processes, such as fermentation, in the absence of oxygen. This adaptability allows them to thrive in a variety of environments, whether oxygen-rich or oxygen-poor.

Other types of organisms mentioned in the choices include aerobes, which require oxygen for growth, and obligate anaerobes, which cannot survive in the presence of oxygen. Microaerophiles, on the other hand, require a lower concentration of oxygen than what is present in the atmosphere for optimal growth. Each of these groups has specific oxygen requirements and limitations, whereas facultative anaerobes are defined by their ability to adapt to varying oxygen levels.