Select the nitrogen oxide which shows disproportionation in alkaline medium:

Disproportionation is a redox reaction in which one substance simultaneously undergoes both reduction and oxidation process. Disproportionation of N₂O₄ taken place in an alkaline medium.

${\stackrel{+4}{\mathrm{N}}}_2{\mathrm{O}}_4+2\mathrm{NaOH}\stackrel{\mathrm{alkaline} \mathrm{medium}}{\to }\mathrm{Na}\stackrel{+3}{\mathrm{N}}{\mathrm{O}}_2+\mathrm{Na}\stackrel{+5}{\mathrm{N}}{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}$

Nitrogen undergoes oxidation when changes N from N₂O₄ to NaNO₃ because oxidation state of N in N₂O₄ is +4 and in NaNO₃ is +5. Nitrogen undergoes reduction when N changes from N₂O₄ to NaNO₂ because oxidation state of N in N₂O₄ is +4 and in NaNO₃ is +3.