Alkyl halide is converted into alkyl isocyanide by reaction with

$\mathrm{AgCN}$ is covalent in nature so it does not dissociate in to ${\mathrm{Ag}}^{+}$ and ${\mathrm{CN}}^{-}$ and on nitrogen electron density is high,and nitrogen is free to donate electron pair forming isocyanide or alkyl isonitrile as the main product.

$\mathrm{R}-\mathrm{X}+\mathrm{AgCN}\to \mathrm{RNC}+\mathrm{AgX}$

$\mathrm{KCN}$ is ionic in nature, so it dissociates completely.

$\mathrm{R}-\mathrm{X}+\mathrm{KCN}\to \mathrm{RCN}+\mathrm{KX}$

Hence the answer is D.