Final answer:
Formaldehyde and acetaldehyde exist to the greatest extent as their hydrates in aqueous solutions due to stabilization through hydrogen bonding.
Step-by-step explanation:
The carbonyl compound that would exist to the greatest extent as its hydrate when dissolved in aqueous solution would be compounds where the carbonyl group is either part of formaldehyde or acetaldehyde. This is due to the presence of hydrogen atoms attached to the carbonyl carbon, which allows greater stabilization of the hydrate form through hydrogen bonding.
According to LibreTexts, exceptions exist where the carbonyl compounds with hydrogen(s) attached (such as in formaldehyde and acetaldehyde) have their hydrate form as the dominant species in the solution. In more substituted carbonyl compounds, steric hindrance and the presence of electron-donating groups decrease the extent of hydration.