Final answer:
Copper (II) ions (Cu²⁺) are most likely to be reduced due to their higher reduction potential compared to Iron (II), Zinc (II), and Nickel (II) ions, making them the correct answer among the given choices.
Step-by-step explanation:
When determining which ion is most likely to be reduced in a given chemical reaction, we look at the reduction potentials of each ion. The species with the higher reduction potential will be more likely to gain electrons and be reduced. Referring to standard reduction potential tables, we can see how different metal ions rank when it comes to their likelihood of being reduced.
Looking at the given options, Fe²⁺, Zn²⁺, Ni²⁺, and Cu²⁺, we can observe their activity in various redox contexts as indicated. For example, in an electrolytic cell, Zinc (Zn) is oxidized at the anode according to Zn(s) → Zn²⁺(aq) + 2e⁻. Conversely, Copper (Cu) is reduced at the cathode: Cu²⁺(aq) + 2e⁻ → Cu(s). This tells us that in a Zinc-Copper cell, Copper (Cu) ions are more likely to be reduced than Zinc (Zn) ions.
Comparing Nickel and Copper, Nickel metal can reduce Copper (II) ions but not Zinc ions, indicating that Copper has a higher tendency to be reduced than Nickel. Thus, Copper (II) ions have a better chance to be reduced than Zinc (II), Nickel (II), or Iron (II) ions.
Given the context and the standard potential values, the correct answer is D. Cu²⁺ because Copper has a higher reduction potential than the other metal ions listed, making it the most likely to be reduced.