Celery stalks that are immersed in freshwater for several hours become stiff. Similar stalks left in a 0.15 M salt solution become limp. From this, we can deduce that the freshwater .

hypertonic to fresh water but hypotonic to the salt solution

Explanation:

When the celery stalks are immersed in the freshwater the cells are  hypertonic to the fresh water this indicate the freshwater is the hypotonic solution. Thus the water moves inside the hypertonic cells and the cells of the celeray stalks becomes hard and stiff due to the development of the turgor pressure inside the cells of the stalks.  

The later, the salt water act as hypotonic for the cells.