Which of the following is a redox reaction ?

$\mathrm{NaCl}+\mathrm{KNO}_3 \rightarrow \mathrm{NaNO}_3+\mathrm{KCl}$
$\mathrm{CaC}_2 \mathrm{O}_4+2 \mathrm{HCl} \rightarrow \mathrm{CaCl}_2+\mathrm{H}_2 \mathrm{C}_2 \mathrm{O}_4$
$\mathrm{Mg}(\mathrm{OH})_2+2 \mathrm{NH}_4 \mathrm{Cl} \rightarrow \mathrm{MgCl}_2+2 \mathrm{NH}_4 \mathrm{OH}$
$\mathrm{Zn}+2 \mathrm{AgCN} \rightarrow 2 \mathrm{Ag}+\mathrm{Zn}(\mathrm{CN})_2$

Solution

The oxidation states show a change only in reaction (d)

Asked in: JEE Main 2002