Which of the following processes occurs without oxygen?

Glycolysis is the metabolic pathway that occurs in the cytoplasm of cells and does not require oxygen, making it an anaerobic process. During glycolysis, glucose is broken down into pyruvate, generating a small amount of ATP and NADH in the process. This pathway is important because it serves as the first step in both aerobic and anaerobic respiration; even in the presence of oxygen, glycolysis takes place prior to further oxidation of pyruvate in aerobic respiration, or fermentation in the absence of oxygen.

In contrast, aerobic respiration, such as the citric acid cycle, requires oxygen to fully oxidize substrates and produce larger amounts of ATP. Photosynthesis, while it does produce glucose, takes place in plants and also depends on light energy, making it distinct from cellular respiration processes. Thus, glycolysis is the only process listed that is exclusively capable of occurring without oxygen.