Identify the compound $[\mathbf{X}]$ in the following reaction.

• A

  $\mathrm{CH}_3 \mathrm{COOH}$
• B

  $\mathrm{OHC}-\mathrm{CHO}$
• C

  $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{OCH}_2 \mathrm{CH}_2 \mathrm{Cl}$
• Correct answer
  D

  $\mathrm{CH}_3 \mathrm{CHO}$

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