Answer: Syringe piston will slide down .
Explanation: If a sealed syringe that contains some gas is plunged unto cold water, now the gas will also gets cool by the effect of cold water.
As the cold gas has a tendency to occupy the less volume, thus making the syringe to move down.
Thus the direction of the syringe piston will be downwards making the way for the cold gas to occupy the lesser volume.