The correct answer is option 1.
Sodium borohydride (NaBH4) is a convenient source of hydride ion (H−) for the reduction of aldehydes and ketones. Ketones are reduced to secondary alcohols. As a source of hydride ion, NaBH4 will also act as a strong base, deprotonating water, alcohols, and carboxylic acids.
The reduction of ketones results in the formation of secondary alcohols. A C−H bond is formed and a C–O (pi) bond is broken.
The mechanism for these reductions follows the very common two-step addition-protonation pattern often found in reactions of aldehydes and ketones.
The first step in this reaction is nucleophilic addition to the carbonyl carbon forming a C-H bond and breaking a C–O (pi) bond.
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