Answer: Oxygen
Explanation:
The final electron acceptor in case of respiration is oxygen. The whole complex consists of many electron acceptor which cannot be substituted.
The first step of cellular respiration is glycolysis and oxygen is not required during this process.
In case of anaerobic respiration, the final electron acceptor is not electron. So here also oxyygen is not required in this process.
Hence, the correct answer is oxygen.
