I have reasons to believe that the answer is unsaturated solution.
When a solution is saturated, it means that it has reached the maximum amount of solute that a solvent can dissolve. Anything before this point will be unsaturated.
Dilution on the other hand is increasing the volume of the solvent, which decreases the concentration of the solute. The question does not talk of decreasing any substance's concentration nor does it talk of increasing the solvent.
It talks about a solution that has not yet reached its saturation point.