In the electron transport chain, the final electron acceptor is:

• A. H₂O.
• B. CO₂.
• C. O₂.
• D. NAD⁺.

Answer: (C)

Verified step by step guidance

1. Understand the context: The electron transport chain (ETC) is the final stage of cellular respiration, occurring in the inner mitochondrial membrane, where electrons are transferred through a series of protein complexes.
2. Identify the role of the final electron acceptor: In the ETC, electrons are passed along the chain and ultimately need to be accepted by a molecule to prevent a backup of electrons.
3. Recognize the importance of oxygen: Oxygen (O2) is the final electron acceptor in the electron transport chain. It combines with electrons and protons to form water (H2O), which is a byproduct of this process.
4. Consider the alternatives: Other molecules like H2O, CO2, and NAD+ are involved in cellular respiration but do not serve as the final electron acceptor in the ETC.
5. Conclude with the correct answer: Based on the role of oxygen in accepting electrons at the end of the electron transport chain, O2 is the correct answer.