Reduction of Oxygen in Methane Combustion

Steps:

• Assign oxidation numbers: In CH₄, C is -4 (4H at +1); in CO₂, C is +4—increase means C oxidized.
• For O: In O₂, 0; in CO₂ and H₂O, -2—decrease means O reduced.
• For H: +1 in CH₄ and H₂O—no change.
• Reaction is redox: C loses electrons (oxidized), O gains electrons (reduced).

Why D is correct:

• Reduction defined as decrease in oxidation number; O goes from 0 to -2, and no other element decreases its number.

Why the others are wrong:

• A: Carbon increases oxidation number (oxidized), but statement ignores that O is reduced in the redox pair.
• B: Carbon's number increases, so it is oxidized, not reduced.
• C: Oxygen decreases oxidation number, so it is reduced, not oxidized.

Final answer: D