Which one of the following reactions is not an example of redox reaction

Check oxidation states. A reaction is not redox if there is no change in oxidation number. In option $\mathrm{A}$, the process is essentially ionization of $\mathrm{HCl}$ in water. The oxidation numbers of $\mathrm{H}$ and $\mathrm{Cl}$ do not change, so it is not a redox reaction. In option $\mathrm{B}$, $\mathrm{Zn}$ goes from $0$ to $+2$ and $\mathrm{Cu}^{2+}$ goes from $+2$ to $0$, so it is redox. In option $\mathrm{C}$, $\mathrm{H}$ goes from $0$ to $+1$ and $\mathrm{O}$ goes from $0$ to $-2$, so it is redox. In option $\mathrm{D}$, sulfur in $\mathrm{SO}_2$ goes from $+4$ to $+6$ and chlorine in $\mathrm{Cl}_2$ goes from $0$ to $-1$, so it is redox. Therefore, the reaction which is not redox is option $\mathrm{A}$.