Which of the following is the major organic product when acetone, CH$_3$COCH$_3$, is allowed to react with hydrogen cyanide in the presence of a catalytic amount of sodium cyanide?

<img src="https://i.sstatic.net/Odv4r.png" width="200">

The correct answer is (B), rather unsurprisingly.  The CN$^-$ attacks the carbonyl, giving rise to a tetrahedral intermediate.  The oxygen anion grabs a proton from HCN, regenerating CN$^-$ and yielding (B).