Which of the following is a redox reaction?

$\stackrel{+}{\mathrm{Na}}\stackrel{-}{\mathrm{Cl}} + \stackrel{+}{\mathrm{K}}{\mathrm{NO}}_3 \to \stackrel{+}{\mathrm{Na}}{\mathrm{NO}}_3 + \stackrel{+}{\mathrm{K}}\stackrel{-}{\mathrm{Cl}}$ ; no change in oxidation state, not a redox reaction.

${\mathrm{CaC}}_2{\mathrm{O}}_4 + 2\mathrm{HCl} \to {\mathrm{CaCl}}_2 + {\mathrm{H}}_2{\mathrm{C}}_2{\mathrm{O}}_4$; no change in oxidation state, not a redox reaction.

$\mathrm{Mg}{\left(\mathrm{OH}\right)}_2 + 2{\mathrm{NH}}_4\mathrm{Cl} \to {\mathrm{MgCl}}_2 + 2{\mathrm{NH}}_4\mathrm{OH}$ ; no change in oxidation state, not a redox reaction.

$\stackrel{0}{\mathrm{Zn}} + 2\stackrel{+1}{\mathrm{Ag}}\stackrel{-1}{\mathrm{CN}} \to 2\stackrel{0}{\mathrm{Ag}} +\stackrel{+2}{ \mathrm{Zn}}\left(\mathrm{CN}\right)2$ ; Zn oxidizes and Ag reduces, Redox reaction.