Final answer:
Oxygen is the final electron acceptor in the electron transport chain during aerobic respiration, where it combines with hydrogen ions to form water, enabling ATP production in cells. Therefore, the correct option is A.
Step-by-step explanation:
The final electron acceptor during the electron transport chain is oxygen, and it accepts two hydrogen ions to form water. The correct answer is a. oxygen; water. Electrons from NADH and FADH2 are passed through protein complexes within the electron transport chain and lose energy, which is used to pump hydrogen ions to create an electrochemical gradient. In the fourth protein complex, terminal electron acceptor oxygen accepts the lower-energy electrons. Oxygen then combines with two hydrogen ions to form water, a critical reaction for the production of ATP during aerobic cellular respiration. In the absence of oxygen, the electron transport chain cannot function, which underscores the importance of breathing in oxygen for energy production in cells.