Paragraph: p-amino-N, N-dimethylaniline is added to a strongly acidic solution of $X$. The resulting solution is treated with a few drops of aqueous solution of $Y$ to yield blue colouration due to the formation of methylene blue. Treatment of the aqueous solution of $Y$ with the reagent potassium hexacyanoferrate(II) leads to the formation of an intense blue precipitate. The precipitate dissolves an excess addition of the reagent. Similarly, treatment of the solution of $Y$ with the solution of potassium hexacyanoferrate(III) leads to a brown colouration due to the formation of $Z$.Question: The compound $X$, is

• A

  $\mathrm{NaNO}_3$
• B

  $\mathrm{NaCl}$
• C

  $\mathrm{Na}_2 \mathrm{SO}_4$
• Correct answer
  D

  $\mathrm{Na}_2 \mathrm{~S}$

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