Why would the cell membrane shrink when the cells were placed in the 4% salt solution?

In a salt solution, the water potential is lower than that in the cell. In this case, water molecules will flow from a region of higher water potential to a region of lower water potential by osmosis. Which where water molecules is now flowing out of the cell to the salt solution. Because the cell lose so much water that it now shrinks.
Osmosis is where water molecules move down the water potential gradient through a semi permeable membrane, which is the cell membrane in this case.

The answer is d.potassium

No because it not possibly