In the absence of oxygen, which process can still occur in both prokaryotic and eukaryotic cells?

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Fermentation is the correct answer because it is a metabolic process that allows cells to generate energy in the absence of oxygen. Both prokaryotic and eukaryotic cells can perform fermentation, utilizing organic molecules to convert sugars into energy, thereby producing byproducts such as ethanol or lactic acid, depending on the organism.

In contrast, aerobic respiration requires oxygen to proceed, so it cannot occur in anoxic conditions. The citrate cycle, also known as the Krebs cycle, is a component of aerobic respiration and is dependent on oxygen availability, making it unsuitable without oxygen. The electron transport chain, which is another part of aerobic metabolism, relies on oxygen as the final electron acceptor, so it similarly cannot function without oxygen. Thus, fermentation remains the only process capable of yielding energy under anaerobic conditions in both types of cells.