The reaction is the Gattermann–Koch reaction, which involves the formylation of benzene to produce benzaldehyde (\(C_6H_5CHO\)). In the presence of carbon monoxide (CO), hydrogen chloride (HCl), and anhydrous AlCl\(_3\)/CuCl, the formyl group (CHO) is introduced into the benzene ring.
\(C_6H_6 + CO + HCl \xrightarrow{\text{AlCl}_3/\text{CuCl}} C_6H_5CHO\)
Thus, the major product \(X\) is benzaldehyde (3).
Thus the correct answer is Option 3.