Which of the structures shown has a larger equilibrium constant for hydration (carbonyl to gem-diol) than the aldehyde shown?

Electron-withdrawing groups destabilise the carbonyl and stabilise the gem-diol. Trichloroethanal (chloral) has a strongly withdrawing CCl3 group and forms a stable crystalline hydrate, so its K for hydration is far larger than for benzaldehyde. The ketones and 4-methoxybenzaldehyde have more stabilised carbonyl groups (alkyl, conjugation or +M donation) and hydrate less.