Answer: B (1 and 4). At the cathode the less reactive species is discharged when a metal cation competes with \(\text{H}^{+}\) from water.
1) Aqueous copper(II) bromide: \(\text{Cu}^{2+}\) is less reactive than hydrogen, so copper metal is deposited. 4) Molten aluminium oxide contains only \(\text{Al}^{3+}\) and \(\text{O}^{2-}\) (no water), so aluminium metal forms at the cathode.
2) and 3) are aqueous sodium chloride (concentrated and dilute): sodium is more reactive than hydrogen, so \(\text{H}_2\) gas is discharged, not a metal. Metals form only in 1 and 4. Answer B.
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