Crystals

The answer is C.

A. can form crystals.
When the solvent of a saturated solution is evaporated, the solution becomes more concentrated than a saturated solution. Then, the excess solute becomes crystals.

B. can form crystals.
When temperature is lowered, the solubility of the solution decreases. Therefore, the solvent cannot dissolve all the solute, and the excess solute becomes crystals.

C. cannot form crystals.
The concentration and solubility do not change when the solution is stirred.

D. can form crystals.
In distillation, when the solvent of a saturated solution is evaporated, the solution becomes more concentrated than a saturated solution. Then, the excess solute becomes crystals.