Answer:
C. It is reduced at the end of the electron transport chain, forming water.
Explanation:
Oxygen is the ultimate acceptor of electron during respiration. It accepts the electron which flows through various complexes of the electron transport chain (ETC) and in the end gets reduced to produce water molecule (H₂O). In the absence of oxygen, aerobic respiration cannot occur because the lack of oxygen will lead to the inhibition of the Krebs cycle. It happens because Krebs cycle needs NAD⁺ and in the absence of oxygen, ETC will not operate so NAD⁺ will not be produced. Thus even if oxygen is not required by Krebs cycle directly, indirectly it is essential for Krebs cycle to operate.
In view of above, we can infer that oxygen is essential for aerobic respiration to occur and in the absence of oxygen only anaerobic respiration which produces less ATP can take place.
