The correct order of reactivity of the following towards nucleophilic addition of $\mathrm{HCN}$ is

Each alkyl group lowers reactivity by its +I effect and by steric crowding in the tetrahedral product. Formaldehyde (no alkyl) is most reactive, then the aldehyde with one methyl, then a ketone with two methyls, and the di-tert-butyl ketone is extremely hindered and least reactive.