Identify the reactant

• A

  $\mathrm{H}_{2} \mathrm{O}$
• B

  $\mathrm{HCHO}$
• Correct answer
  C

  $\mathrm{CO}$
• D

  $\mathrm{CH}_{3} \mathrm{CHO}$

Solution

When a mixture of $\mathrm{CO}$ and $\mathrm{HCl}$ gas is passed through benzene in presence of catalyst consisting anhydrous $\mathrm{AlCl}_{3}$, benzaldehyde is formed. $\mathrm{CO}+\mathrm{HCl} \rightarrow \mathrm{HOCl}$ $\quad\quad\quad\quad\quad\quad$ formyl chloride $\mathrm{C}_{6} \mathrm{H}_{6}+\mathrm{HOCl} \stackrel{\mathrm{AlCl}_{3}}{\longrightarrow} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}+\mathrm{HCl}$

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