Assertion (A): ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{Cl}$ gives ethyl cyanide as a major product with alc $\mathrm{KCN}$ but ethyl iso-cyanide as a major product with alc $\mathrm{AgCN}$.

Reason (R): $\mathrm{KCN}$ is ionic compound but $\mathrm{AgCN}$ is covalent.

Reaction of ethyl chloride with $\mathrm{KCN}$,

                 ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{Cl}+\mathrm{KCN}\to {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{CN}+\mathrm{KCl}$

Being ionic in nature, $\mathrm{KCN}$ gives ${\mathrm{K}}^{+}$and ${\mathrm{CN}}^{-}$. This ${\mathrm{CN}}^{-}$attack as a nucleophile from the carbon side to the ethyl chloride to give an alkyl cyanide as a final product.

Reaction of ethyl chloride with $\mathrm{AgCN}$,

${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{Cl}+\mathrm{AgCN}\to {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{NC}+\mathrm{AgCl}$

Being covalent in nature, $\mathrm{AgCN}$ does not give ${\mathrm{Ag}}^{+}$and ${\mathrm{CN}}^{-}$. So, $\mathrm{AgCN}$ attack as a nucleophile from the nitrogen side to the ethyl chloride to give an alkyl isocyanide as a final product.

Thus, both assertion and reason are true and reason is the correct explanation of assertion

Hence, option (A) is correct.