This process is used in the absence of oxygen to release energy.

• A. Respiration
• B. Fermentation
• C. Photosynthesis
• D. Digestion

Answer: (B)

When oxygen isn’t available, fermentation is the way cells release energy. It’s an anaerobic process that lets glycolysis keep running by regenerating the molecules (like NAD+) needed to keep splitting glucose. Because it skips the full set of steps used in oxygen-based respiration, fermentation produces only a small amount of ATP compared with aerobic respiration. This is why it’s useful for short bursts of activity or in organisms like yeast, which use fermentation to make bread and alcoholic beverages. In muscles, lactic acid fermentation can supply quick energy during intense exercise, though it can lead to fatigue from buildup of lactic acid. Photosynthesis builds energy-storing sugar using light, not releasing energy without oxygen, and digestion is about breaking down food for nutrients rather than generating cellular energy.