The best reagent to accomplish the following transformation is a) NaBH4. Therefore, a) NaBH4 is correct .
NaBH4 is a mild reducing agent that is selective for aldehydes and ketones.
It will reduce the aldehyde in the reactant to a primary alcohol in the product.
The other reagents on the list are either too strong (LiAlH4) or not selective enough (BH3, H2SO4, H2/Pd).
Here is a mechanism for the reduction of aldehydes with NaBH4:
NaBH4 reacts with the aldehyde to form a tetrahedral intermediate.
The intermediate is protonated by water to form a hemiaminal.
The hemiaminal collapses to form the alcohol and boric acid.
The reaction is typically carried out in a protic solvent, such as ethanol or methanol.
The solvent helps to protonate the intermediate and stabilize the boric acid product.
Here is an example of a reduction of an aldehyde with NaBH4:
CH3CHO + NaBH4 + H2O -> CH3CH2OH + NaBO2
NaBH4 is a versatile and widely used reducing agent in organic synthesis.
It is relatively inexpensive and easy to use.
It is also relatively safe, although it is important to take precautions when handling any strong reducing agent.