Nitrogen reduction via decreased oxidation state Steps:

• Calculate oxidation state of N in reactants and products for each reaction.
• Reduction occurs when N's oxidation state decreases.
• In option A, N in NO is +2 (O is -2); in N₂, N is 0—decreases to 0.
• Options B–D show no decrease or increase in N's oxidation state.

Why A is correct:

• N's oxidation number decreases from +2 to 0, defining reduction as a loss of oxidation number (or gain of electrons).

Why the others are wrong:

• B: N's oxidation state stays +2 in NO and NH₃ (no change).
• C: N's oxidation state increases from +4 in NO₂ to +2 in NO (oxidation).
• D: N's oxidation state increases from +4 in NO₂ to -3 in NH₃ (but overall oxidation due to H₂).

Final answer: A