Preparation of Aldehydes

Diisobutylaluminium hydride (DIBAL-H, $[(i\text{Bu})_2\text{AlH}]$) is a bulky, selective reducing agent. At low temperatures (around $-78^\circ\text{C}$), it reduces esters cleanly to aldehydes upon subsequent aqueous workup without reducing them further to primary alcohols. In contrast, $\text{LiAlH}_4$ reduces esters completely to primary alcohols, $\text{NaBH}_4$ is generally unreactive toward esters, and Rosenmund catalyst ($\text{H}_2/\text{Pd}-\text{BaSO}_4$) selectively reduces acyl chlorides, not esters.

Benzonitrile with $CH_3MgBr$ followed by hydrolysis gives acetophenone ($C_6H_5COCH_3$), a ketone, not benzaldehyde. Benzoyl chloride with $H_2$/Pd-$BaSO_4$ (Rosenmund reduction), ethyl benzoate with DIBAL-H, and toluene with $CrO_2Cl_2$ (Etard reaction) all give benzaldehyde.

Zn–Hg and conc. HCl reduce aldehydes and ketones, but a carboxylic group remains unaffected, so benzoic acid does not give benzaldehyde.

An acid chloride is reduced by $H_2$ over palladium supported on $BaSO_4$ to an aldehyde. This is the Rosenmund reduction.
$BaSO_4$ poisons the catalyst so that the aldehyde is not reduced further to the alcohol.

So A is benzaldehyde, $C_6H_5CHO$.