Which of the following is a redox reaction?

• A

  $\mathrm{NaCl}+\mathrm{KNO}_3 \longrightarrow \mathrm{NaNO}_3+\mathrm{KCl}$
• B

  $\mathrm{Mg}(\mathrm{OH})_2+2 \mathrm{NH}_4 \mathrm{Cl} \longrightarrow \mathrm{MgCl}_2+2 \mathrm{NH}_4 \mathrm{OH}$
• C

  $\mathrm{CaC}_2 \mathrm{O}_4+2 \mathrm{HCl} \longrightarrow \mathrm{CaCl}_2+\mathrm{H}_2 \mathrm{C}_2 \mathrm{O}_4$
• Correct answer
  D

  $\mathrm{Zn}+2 \mathrm{AgCN} \longrightarrow 2 \mathrm{Ag}+\mathrm{Zn}(\mathrm{CN})_2$

Hence, the correct answer is option: 'D'.