Answer: N (the Nitrogen)
Step-by-step explanation:
Reduction refers to a decrease in oxidation number/state due to the gaining of electrons. As such the species that is being reduced will show a decrease in oxidation state.
Based on the redox rules,
Zn(s) has oxidation number of 0 [rule 1: the oxidation number of an element in its free (uncombined) state is zero]
Zn²⁺ has oxidation number of +2 [rule 2: The oxidation number of a monatomic (one-atom) ion is the same as the charge on the ion]
Now, since Nitrogen is enbedded in a polyatomic ion in both cases, you have to do a bit a calculation to obtain the oxidation state.
For NO₃⁻ : N + (-2 × 3) = -1
N - 6 = -1
N = 5
[Rule 3: The sum of all oxidation numbers in a polyatomic (many-atom) ion is equal to the charge on the ion; Rule 6: The oxidation state of hydrogen in a compound is usually +1]
For NH₄⁺ :
N + (4 x 1) = 1
N + 4 = 1
N = -3
[Rule 3: The sum of all oxidation numbers in a polyatomic (many-atom) ion is equal to the charge on the ion; Rule 5: The oxidation number of oxygen in a compound is usually –2]
Therefore, Zn moves from oxidation state of 0 to +2 (oxidation), while N moves from +5 to -3 (reduction).