$\mathrm{H}\mathrm{C}\mathrm{N}+{\mathrm{H}}_2\mathrm{O}\to \left[\mathrm{X}\right]+\mathrm{N}{\mathrm{H}}_3\text{,}$

$\left[\mathrm{X}\right]\to \left[\mathrm{Y}\right]+{\mathrm{H}}_2\mathrm{O}\mathrm{ }\left(\mathrm{U}\mathrm{n}\mathrm{b}\mathrm{a}\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{d}\mathrm{ }\mathrm{e}\mathrm{q}\mathrm{u}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}\right)$

[Y] May be prepared by which one of the following methods?

• A

  By dehydration of Malonic acid with ${\mathrm{P}}_2{\mathrm{O}}_5$
• B

  By thermal decomposition of carbon sub-oxide in air.
• Correct answer
  C

  By heating potassium hexacyanoferrate (II) with conc. ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ .
• D

  By the action of conc. $\mathrm{H}\mathrm{N}{\mathrm{O}}_3$ on charcoal.

Hence, the correct answer is option: 'C'.