Which process occurs when oxygen is not present?

Anaerobic fermentation occurs in the absence of oxygen, making it the correct answer to the question. This process allows cells to generate energy without relying on the oxygen that is typically required for aerobic respiration. During anaerobic fermentation, glucose is partially broken down to produce energy, along with byproducts such as lactic acid in animals or ethanol and carbon dioxide in yeast. This process is vital in environments where oxygen is scarce and allows organisms to survive and generate energy under such conditions.

In contrast, aerobic respiration and cellular respiration require oxygen to effectively harvest energy from glucose, while anabolic metabolism refers to the set of metabolic pathways that construct molecules from smaller units, which can occur with or without oxygen but is not specifically focused on energy generation in absence of oxygen.