The answer is (1)
In the cell of the body, oxygen is used in a process that releases energy. Oxygen is used in the last and most important step in cellular respiration namely the electron transport chain.
In this process electrons are passed from one electron carrier to another and as they are thus transferred, energy is released. Toward the final step, electrons are transferred to oxygen which is the final acceptor of the electrons. By the time the process ends, 34 ATP molecules are produced.