Which of the following forms of bacteria can thrive in an oxygen-free environment?

Bacteria that can thrive in an oxygen-free environment are classified as anaerobic bacteria. Anaerobic bacteria do not require oxygen for growth and may even find oxygen toxic. They obtain energy through processes such as fermentation or anaerobic respiration, utilizing alternative electron acceptors like nitrates, sulfates, or carbon dioxide.

Facultative bacteria, while they can grow in both the presence and absence of oxygen, do not exclusively thrive in an oxygen-free environment. They can switch between aerobic and anaerobic respiration based on the availability of oxygen. Conversely, aerobic bacteria require oxygen to grow and thrive, making them unsuitable for oxygen-free settings.

Thus, the correct answer focuses specifically on anaerobic bacteria, which are uniquely adapted to survive and function in environments devoid of oxygen.