Final answer:
Sodium (Na+) is the least likely to be reduced because it has a strong effective nuclear charge and a small atomic radius, indicating its reluctance to accept electrons and undergo reduction. Option a.
Step-by-step explanation:
The least likely to be reduced among Na+, Ni, Cu, and Fe2+ can be deduced by examining their standard reduction potentials. In general, elements with more positive standard reduction potentials are more likely to be reduced, meaning they are stronger oxidizing agents. In the case of Na+, we recognize that it has a much more positive effective nuclear charge and a small atomic radius, which makes it less likely to gain electrons.
Given the provided electrochemical data, Na+ is the least likely to be reduced since metallic sodium is a very strong reducing agent and its ion, Na+, has a high ionization energy, reflecting its reluctance to accept electrons and be reduced.