Final answer:
The electron transport chain occurs in the presence of oxygen, while glycolysis occurs in the absence of oxygen.
Step-by-step explanation:
The process that occurs in the presence of oxygen is the electron transport chain. This is the portion of aerobic respiration that uses free oxygen as the final electron acceptor to generate ATP. The electrons are passed through a series of chemical reactions in the electron transport chain, and the end products are water and ATP.
In the absence of oxygen, glycolysis is the process that occurs. Glycolysis is an anaerobic process that takes place in the cytoplasm of cells and breaks down glucose to produce pyruvate. Although glycolysis can proceed without oxygen, it produces only a small amount of ATP compared to aerobic respiration.Therefore, the correct answer is A. Electron transport chain (occurring in the presence of oxygen) and D. Glycolysis (occurring in the absence of oxygen).