In the absence of oxygen, what process can cells undergo to produce energy?

In the absence of oxygen, cells can undergo fermentation to produce energy. This process allows cells to generate ATP through the partial breakdown of glucose without utilizing oxygen. Fermentation can occur in various organisms and results in different end products, such as lactic acid in muscles or ethanol in yeast. It is particularly crucial for cells that either lack mitochondria or find themselves in anaerobic conditions, as it provides a means to continue generating energy when aerobic respiration, which requires oxygen, is not possible.

Aerobic respiration, by contrast, relies on oxygen to fully oxidize glucose and is significantly more efficient in terms of ATP yield. Photosynthesis involves sunlight, water, and carbon dioxide to create glucose and oxygen, not energy production in anaerobic conditions. Photorespiration is a process that occurs in plants alongside photosynthesis but does not directly relate to energy production in the absence of oxygen. Thus, fermentation is the only viable pathway from the options given for energy production when oxygen is lacking.