In the reaction,
$A$ is

• A

  $\mathrm{HgSO}_4 / \mathrm{H}_2 \mathrm{SO}_4$
• B

  $\mathrm{Cu}_2 \mathrm{Cl}_2$
• Correct answer
  C

  $\mathrm{H}_3 \mathrm{PO}_2$ and $\mathrm{H}_2 \mathrm{O}$
• D

  $\mathrm{H}^{+} / \mathrm{H}_2 \mathrm{O}$

Solution

When diazonium salt is treated with $\mathrm{H}_3 \mathrm{PO}_2$ followed by hydrolysis, it diazonium group is replaced by $-\mathrm{H}$ resulting to the formation of hydrocarbon. Thus, $A$ must be $\mathrm{H}_3 \mathrm{PO}_2 / \mathrm{H}_2 \mathrm{O}$.

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