Ethyl chloride on heating with silver cyanide forms a compound ‘X’. The functional isomer of ‘X’ is

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

Isocyanide serves as the functional group in this reaction because silver and carbon have a partially covalent bond. Cyanide ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{CN}$ is a functional isomer of isocyanide ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{NC}$ (X). For the synthesis of other chemicals, they serve as the building blocks.

Hence, the correct answer is option (B) ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{CN}$.