Which of the following carbonyl compounds have no alpha hydrogen and therefore cannot form an enolate?

An enolate needs an sp3 C-H on the carbon next to $\mathrm{C}=\mathrm{O}$. In methanal the carbonyl carbon carries only H. In benzaldehyde the neighbour is an aromatic carbon with no H to lose. In 2,2-dimethylpropanal the neighbour is a quaternary carbon. Ethanal has three alpha hydrogens, so it is excluded.