Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R .…

$\mathrm{KCN}$ is ionic in nature and provides cyanide ions in solution. Although both carbon and nitrogen atoms are in a position to donate electron pairs, the attack takes place mainly
through carbon atom and not through nitrogen atom since $\mathrm{C}-\mathrm{C}$ bond is more stable than $\mathrm{C}-\mathrm{N}$ bond.
Involved reaction is shown below:
$$\begin{gathered} \mathrm{R}-\mathrm{X} \stackrel{}{\stackrel{\mathrm{Substitution} \mathrm{reaction}}{\to }\mathrm{R}-\mathrm{CN}+\mathrm{KX}} \\ \mathrm{Alkyl} \mathrm{halide} \mathrm{Cyanide} \end{gathered}$$
Reaction of alkyl halide with $\mathrm{AgCN}$
$\mathrm{AgCN}$ is mainly covalent in nature. Here only nitrogen is free to donate electron pairs,hence it forms isocyanide as the main product.
Involved reaction is shown below:
$$\begin{gathered} \mathrm{R}-\mathrm{X}+\mathrm{AgCN}\stackrel{\mathrm{Substitution} \mathrm{reaction}}{\to }\mathrm{R}-\mathrm{NC}+\mathrm{AgX} \\ \mathrm{Alkyl} \mathrm{halide} \mathrm{Isocyanide} \end{gathered}$$
Hence, here isocyanide is formed as major product.

$\mathrm{AgCN}$ is mainly covalent in nature and nitrogen is available for attack, so alkyl isocyanide is formed as main product.

Cyanide ion is an ambidentate ligand (nucleophile or lewis base). It means that donation of a pair of electrons can be through either carbon or nitrogen, so it has two nucleophilic centres. $\mathrm{KCN}$ is predominantly ionic and $\mathrm{AgCN}$ is predominantly covalent.