Ans. (C)

Explanation:

(C) ${\mathrm{N}}_2+{\mathrm{O}}_2\stackrel{2000 \mathrm{K}}{\to }2\mathrm{NO} \mathbf{:}$ The oxidation state of N changes from 0 to +2, and the oxidation state of O changes from 0 to -2. It means this reaction is redox reaction.

In all the remaining reactions, there is no change in oxidation states of the elements participating in the reaction.

(A) 3O₂ → 2O₃

(B) 2NaOH + H₂SO₄ →  Na₂SO₄ + 2H₂O (Neutralisation reaction)

(D) 3 AgNO₃ + [Co(H₂O)₆]Cl₃ → [Co(H₂O)₆](NO₃)₃ + 3 AgCl (Double displacement)