In the Cannizzaro reaction: $2\text{PhCHO}\stackrel{:{\text{OH}}^{-}}{\to }{\text{PhCH}}_2\text{OH}+{\text{PhCOO}}^{-}$, the slowest step is:

The mechanism of the Cannizzaro reaction involves:Nucleophilic attack of ${\text{OH}}^{-}$ on the carbonyl carbon (fast).Hydride ion (${\text{H}}^{-}$) transfer from the resulting anion to another aldehyde molecule.The hydride transfer is the rate-determining step (slowest step) because ${\text{H}}^{-}$ is a poor leaving group and a very strong base.