Alkyl halide is converted into alkyl isocyanide by reaction with

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

$R - X + AgCN \to RNC + AgX$

$KCN$ is ionic in nature, so it dissociates completely.

$R - X + KCN \to RCN + KX$

Hence the answer is D.