To determine which method cannot prepare Benzaldehyde ($C_6H_5CHO$), we review the standard preparation methods listed in the NCERT textbook:

1. Etard Reaction: Oxidation of toluene with chromyl chloride ($CrO_2Cl_2$) yields a chromium complex which, upon hydrolysis, gives benzaldehyde. (Valid Method)
2. Gattermann-Koch Reaction: Benzene is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride ($AlCl_3$) to give benzaldehyde. (Valid Method)
3. Side-chain Chlorination: Toluene is chlorinated to give benzal chloride ($C_6H_5CHCl_2$), which upon acid hydrolysis gives benzaldehyde. (Valid Method)
4. Rosenmund Reduction: Hydrogenation of benzoyl chloride over $Pd-BaSO_4$ gives benzaldehyde. (Valid Method)

Why Option 4 is the likely answer: A common incorrect method presented in this context is the reaction of Benzoic Acid with Zinc Amalgam (Zn-Hg) and conc. HCl. This reagent (Clemmensen reduction) is used to reduce the carbonyl group ($>C=O$) of aldehydes and ketones to a methylene group ($-CH_2-$). It does not reduce carboxylic acids to aldehydes. Similarly, oxidation of toluene with strong oxidizing agents like $KMnO_4$ yields benzoic acid, not benzaldehyde.