Explanation

(d)
Benzaldehyde when heated with ethanolic $KCN$, it gives $\alpha -$hydroxy ketone, benzoin.
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ O$
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$C_{6}H_{5}CHO + H - C - C_{6}H_{5}\underset{\mathrm{\Delta}}{\overset{\ \ \ Alc.KCN\ \ \ \ }{\rightarrow}}$
Benzaldehyde (2 mol)
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ O$
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${\ \ \ \ \ \ \ C}_{6}H_{5}CH(OH)C - C_{6}H_{5}$
benzoin