Which of the following forms an acetal with methanol and dry $\mathrm{HCl}$ most readily (all other conditions identical)?

Acetal formation is favoured for small, unhindered and electrophilic carbonyl carbons. The aldehyde has only one alkyl group and gives the least crowded tetrahedral product, while the ketones are more hindered and thermodynamically less favourable, especially the di-tert-butyl ketone.