The correct answer to the question is option 3, oxygen. Respiration is a biological process that occurs in living organisms to produce energy. It involves the breakdown of glucose molecules in the presence of oxygen to release energy, carbon dioxide, and water. Oxygen is essential for respiration as it serves as the final electron acceptor in the electron transport chain, which produces adenosine triphosphate (ATP), the energy currency of cells. Without oxygen, the process of respiration cannot proceed, resulting in a lack of energy production and eventually cell death. Therefore, option 3 is the correct answer as respiration requires the presence of oxygen.