The Rosenmund reaction is a specific hydrogenation process where an acyl chloride (acid chloride) is converted into an aldehyde. This reduction is carried out using hydrogen gas ($H_2$) in the presence of a catalyst, palladium supported on barium sulphate ($Pd/BaSO_4$). The reaction scheme is generally represented as: $R-COCl + H_2 \xrightarrow{Pd/BaSO_4} R-CHO + HCl$

Other options refer to different preparations:

• Stephen reaction: Reduction of nitriles to aldehydes using $SnCl_2/HCl$.
• Gattermann-Koch reaction: Formylation of benzene derivatives using $CO$ and $HCl$ with $AlCl_3$.
• Etard reaction: Oxidation of the methyl group in toluene to an aldehyde using chromyl chloride ($CrO_2Cl_2$).