Electrolysis of molten metal oxides deposits pure metal at the cathode Steps:

• Identify cathode reaction: metal ions (Al³⁺, Cu²⁺, Pb²⁺, Mg²⁺) gain electrons to form neutral metal atoms.
• Confirm for all oxides: molten state ensures no water, so only metal ions reduce at cathode, yielding pure metal.
• Evaluate options: A fails for reactive metals, C ignores varying equivalent weights, D uses inert electrodes.

Why B is correct:

• Faraday's laws define cathode as reduction site; in molten oxides, Mⁿ⁺ + n e⁻ → M produces pure metal without impurities.

Why the others are wrong:

• A: Aluminium and magnesium oxides are too stable for carbon reduction with coke; require electrolysis.
• C: Mass m = (M/n) × (Q/F); different atomic masses M and valencies n yield different masses for same charge Q.
• D: Inert electrodes (e.g., graphite) prevent anode dissolution; metal electrodes would react undesirably.

Final answer: B