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?

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).