The rate of cyanohydrin formation from acetone and HCN is very small in pure HCN but increases when a few drops of NaOH are added; it falls again if strong acid is added. This is because

1. Abase converts acetone to its enol which attacks HCN
2. Bbase increases the concentration of the nucleophile $\mathrm{CN}{}^{-}$ and acid suppresses it
3. Cbase protonates the carbonyl oxygen and acid generates $\mathrm{CN}{}^{-}$
4. Dacid catalyses direct attack of molecular HCN on carbon