Question: Which of the following does not form a stable hydrate?
I think they mean nucleophilic addition of water. For compound (C), on reaction with water, the product formed is
The product has lesser bond angle strain than the reactant. Hence, the compound formed is stable. This is because $\ce{sp^2}$ hybridised carbonyl carbon requires $120^{\circ}$ but only gets $60^{\circ}$ in cyclopropanone. But converting it into a diol makes the angle $109^{\circ}$ which reduces strain to a large extent. In fact, cyclobutanone is also reactive to water.
For (B) however, there is not much change in the bond angle strain. The initial angle is already close to the required angle.
What can I say about (A) and (D)?