Final answer:
Obligate anaerobes are organisms that are poisoned by oxygen and must live under low oxygen conditions, as they lack defenses against oxidative stress and use anaerobic respiration or fermentation. The correct option is b anaerobes.
Step-by-step explanation:
Organisms that are poisoned by oxygen and must live under very low oxygen conditions are known as obligate anaerobes. These microorganisms utilize anaerobic respiration or fermentation to survive because oxygen can be lethal for them.
They lack the cellular defenses, such as enzymes, that protect against oxidative stress caused by oxygen free radicals. In contrast, facultative anaerobes can survive with or without oxygen, relying on aerobic metabolism when oxygen is present and switching to anaerobic metabolism when it is not.
Aerotolerant organisms can tolerate the presence of oxygen but do not use it for energy production, while obligate aerobes require oxygen to live as they use it as a terminal electron acceptor during cellular respiration.