Identify the transformation shown by the named reaction

The reaction referred to in this question is the formylation of an aromatic ring using carbon monoxide and hydrogen chloride in the presence of a catalyst such as anhydrous $\mathrm{AlCl_3/CuCl}$.

This introduces a $-CHO$ (aldehyde) group onto an aromatic ring:

$\mathrm{Ar-H \xrightarrow[AlCl_3/CuCl]{CO + HCl} Ar-CHO}$

Match with the correct name reaction

This reaction is known as the Gattermann–Koch reaction.

Check the other options

• A: Stephen Reaction
  Converts nitriles into aldehydes by reduction followed by hydrolysis.
• B: Etard Reaction
  Oxidation of a methyl group on an aromatic ring to an aldehyde using chromyl chloride.
• C: Gatterman - Koch Reaction
  Introduces a formyl group into an aromatic ring using $\mathrm{CO}$ and $\mathrm{HCl}$. ✅
• D: Rosenmund Reduction
  Converts acid chlorides into aldehydes using poisoned palladium catalyst.

Conclusion

The reaction is Gattermann–Koch Reaction.

Hence, the correct option is:

$\boxed{\text{C}}$