Which is least likely to be reduced?
A. Zn^2+
B. Fe^3+
C. Cu^2+
D. Fe^2+

A. Zn²⁺

Further explanation

Given

Cations of several elements

Required

The least to be reduced

Solution

If we look at the voltaic series:

Li-K-Ba-Ca-Na-Mg-Al-Mn- (H2O) -Zn-Cr-Fe²⁺-Cd-Co-Ni-Sn-Pb- (H) -Cu-Hg-Fe³⁺-Ag-Pt-Au

The electrode which is easier to reduce than the hydrogen (H2) electrode has a positive sign (E red= +) and is located to the right of the voltaic series (right of H)

The electrode which is easier to oxidize than the hydrogen (H2) electrode and is difficult to experience reduction has a negative sign (E red= -) and is located to the left of the voltaic series (left of H)

Or you can look at the standard reduction potential value of the metals in the answer options, and the most negative reduction E° value which will be difficult to reduce.

The Zn metal is located far left of the other metals in the answer choices, so it is the most difficult to reduce

Now, you know that the sample of sodium metal reacted completely, which implies that it was completely consumed by the reaction.
In other words, you don't have to worry about the sample of chlorine gas because the fact that sample of sodium metal was completely consumed lets you know that the chlorine gas is not a limiting reagent.
This means that the reaction consumed