Answer: D. A displacement reaction \(\text{metal} + \text{metal oxide} \rightarrow \text{new oxide} + \text{new metal}\) proceeds only when the free metal on the left is MORE reactive than the metal in the oxide. If it's less reactive, the reaction does not occur.
Reactivity series (relevant part): K > Na > Ca > Mg > Al > C > Zn > Fe > Pb > H > Cu > Ag.
A: \(\text{Mg} + \text{Ag}_2\text{O}\) — Mg is far more reactive than Ag. Occurs readily.
B: \(\text{Zn} + \text{Fe}_2\text{O}_3\) — Zn is more reactive than Fe. Occurs.
C: \(\text{Ca} + \text{MgO}\) — Ca is more reactive than Mg. Occurs.
D: \(\text{Cu} + \text{PbO}\) — Cu is less reactive than Pb. Cu cannot displace Pb from its oxide. The reaction does NOT occur.
So D is the least likely (effectively impossible) — the answer.
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