What is a defining feature of fermentation processes?

Fermentation processes are characterized by their ability to occur in the absence of oxygen, which makes option C the defining feature of fermentation. During fermentation, organisms convert sugars into energy through anaerobic pathways, allowing them to regenerate NAD+ from NADH. This regeneration is crucial because it enables glycolysis to continue producing ATP, even without oxygen.

In the context of cellular respiration, the presence of oxygen typically allows for aerobic metabolism, which is more efficient in generating ATP. However, under anaerobic conditions, fermentation serves as an alternative pathway that supports energy production when oxygen is scarce.

The other options do not correctly define fermentation. Fermentation does not require oxygen; rather, it explicitly occurs under anaerobic conditions. Additionally, it does not produce glucose as a primary product; instead, glucose is broken down into other byproducts like lactate or ethanol. Finally, fermentation is distinct from aerobic respiration, which generates ATP primarily through oxidative phosphorylation in the presence of oxygen, rather than through the less efficient substrate-level phosphorylation seen in fermentation processes.