Effect of isotopes on H-Bonding
Hydration of acetone is an equilibrium process.
$$\ce{(CH3)2C^16O + H2^18O &lt;=&gt; (CH3)2C^18O + H2^16O }$$
The equilibrium constant of this reaction will depend on how the bond strengths of the $\ce{O-H}$ bond in water and the $\ce{O=C}$ bond in acetone is affected by isotope exchange. The isotope-dependent strength of hydrogen bonds might be a secondary effect, as well.
The ratio of heavy to light acetone will not only be dependent on the equilibrium constant, but also on the ratio of acetone to heavy water initially present.
So none of the statements are sufficient to explain the observation completely. Statements (a) and (b) are correct. Statement (c) is not directly related to the ratio of the species. You also have to consider the isotope effect on the strength of hydrogen bonds with water (as one acetone changes from light to heavy, one water changes from heavy to light).