Haloalkanes ($\text{R-X}$) react with $\text{AgCN}$ to form alkyl isocyanides (isonitriles) as the main product. $\text{AgCN}$ is mainly covalent in nature, and only the nitrogen atom is free to donate an electron pair, thus forming an isocyanide. On the other hand, $\text{KCN}$ and $\text{NaCN}$ are predominantly ionic and provide cyanide ions in solution. Since the carbon-carbon ($\text{C-C}$) bond is more stable than the carbon-nitrogen ($\text{C-N}$) bond, the attack takes place mainly through the carbon atom, leading to the formation of alkyl cyanides (nitriles) . Therefore, only reaction (B) generates isonitriles.