Step 1: Identify the reagent.
$Zn(Hg)/\text{conc. HCl}$ is Clemmensen's reagent, which reduces the carbonyl group ($C{=}O$) in aldehydes and ketones to a methylene or methyl group, converting them to the corresponding hydrocarbon.
Step 2: Starting compound and the change.
Benzaldehyde ($C_6H_5CHO$) has a $-CHO$ group directly on the benzene ring. Clemmensen reduction replaces the oxygen of $-CHO$ with two hydrogens, converting it to $-CH_3$.
Step 3: Product formed.
\[ C_6H_5CHO \xrightarrow{Zn(Hg)/\text{conc. HCl}} C_6H_5CH_3 \] The product is toluene ($C_6H_5CH_3$), formed by complete reduction of the aldehyde carbonyl group.