The redox reaction among the following is

Redox reaction = reaction involving change in oxidation states.

Step 1 — Analyse each option:

(1) Formation of ozone from O₂: $\mathrm{3O_2 \xrightarrow{h\nu} 2O_3}$ O is $0$ in $\mathrm{O_2}$ and $0$ in $\mathrm{O_3}$ (average). No change in oxidation state → Not redox ✗

(2) $[\mathrm{Co(H_2O)_6}]\mathrm{Cl_3}$ with $\mathrm{AgNO_3}$: $[\mathrm{Co(H_2O)_6}]\mathrm{Cl_3} + 3\mathrm{AgNO_3} \rightarrow 3\mathrm{AgCl} + [\mathrm{Co(H_2O)_6}](\mathrm{NO_3})_3$ This is a double displacement/precipitation reaction. No change in oxidation states → Not redox ✗

(3) $\mathrm{H_2SO_4}$ with NaOH: $\mathrm{H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O}$ Acid-base neutralisation. No change in oxidation states → Not redox ✗

(4) $\mathrm{N_2 + O_2 \rightarrow 2NO}$ at 2000 K:

• N: $0 \rightarrow +2$ (oxidised)
• O: $0 \rightarrow -2$ (reduced) Change in oxidation states → Redox reaction ✓

Answer: Option (4)