Rusting requires O₂ and H₂O; magnesium sacrifices to protect iron

Steps:

• Rusting is oxidation of iron needing both oxygen and water for electrochemical reaction.
• Sacrificial protection uses a more reactive metal like magnesium, which corrodes preferentially.
• Check options: conditions must include O₂ + H₂O, and metal must be more reactive than iron.
• Option D matches both criteria.

Why D is correct:

• Rusting follows 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃, requiring O₂ and H₂O; magnesium, above iron in reactivity series, acts as anode in galvanic cell, oxidizing instead (Mg → Mg²⁺ + 2e⁻).

Why the others are wrong:

• A: O₂ alone insufficient without H₂O; copper below iron in reactivity series, cannot sacrifice.
• B: O₂ alone insufficient; magnesium correct for protection but conditions incomplete.
• C: H₂O alone insufficient without O₂; copper cannot provide sacrificial protection.

Final answer: D