Step 1: Identifying the reaction.
The reaction of benzene with carbon monoxide and hydrogen chloride in presence of anhydrous \( \mathrm{AlCl_3} \) and \( \mathrm{Cu_2Cl_2} \) is known as the Gattermann–Koch reaction.
Step 2: Reaction outcome.
In this reaction, a formyl group (\(-CHO\)) is introduced into the benzene ring, resulting in the formation of benzaldehyde.
Step 3: Elimination of incorrect options.
Toluene and acetophenone are alkylated products, while benzoic acid is formed through oxidation, not this reaction.
Step 4: Conclusion.
Therefore, the correct product obtained is benzaldehyde.