The enzyme would also be expected to catalyse reaction C since the only difference is that benzaldehyde has been replaced by the closely related substrate para-methylbenzaldehyde. Provided that this small change does not prevent the aldehyde from fitting into the active site of the enzyme (and in fact it will fit), then this aldehyde will also be a substrate for the reaction and will give the enantiomer of the product shown in reaction C.
The enzyme will not catalyse the reaction shown in reaction D. The aldehyde is a suitable substrate for the enzyme, but the enzyme gives the opposite enantiomer of the product to that shown in reaction D. The enzyme will always catalyse the addition of HCN to the si-face of the aldehyde as in reactions B and C rather than the re-face as would be required for reaction D.
The main advantage of using the synthetic catalyst E is that both enantiomers of the catalyst can be prepared, and hence both enantiomers of a cyanohydrin product could be obtained. Hence, the enantiomer of compound E would be a suitable catalyst for reaction D, a reaction which cannot be catalysed by the oxynitrilase enzyme.