Final answer:
When O₂ accepts electrons in the electron transport chain, it becomes reduced, transforming into water as it gains electrons from NADH and FADH₂ which are oxidized in the process.
Step-by-step explanation:
When O₂ accepts electrons in the electron transport chain, O₂ becomes reduced. This process occurs during the final stages of cellular respiration where electrons are removed from NADH and FADH₂ and ultimately transferred to oxygen, thereby reducing it into water (H₂O). The overall reaction can be described as O₂ + 4e⁻ + 4H+ → 2 H₂O, where oxygen acts as the terminal electron acceptor and is reduced by electrons resulting from the oxidation of NADH and FADH₂ to their oxidized forms, NAD+ and FAD.