When a cell shrinks in size, what type of solution was added to it

A hypertonic solution was added to it and it leads to the cell shrinking.

Hypertonic means the solution has a lower water potential than the cell, and because of osmosis, water molecules will flow from a region of higher water potential to a region of lower water potential, through the semi permeable membrane, which is the cell membrane. Therefore, the water will flow away from the cell. This process is caused by natural tendency. The cell lose so much water that the cell shrink.

Note that only animal cells shrink if put in hypertonic solution, plant cell does not since it has a cell wall. In plant cells, cell membrane will detach from the cell wall and plasmolysis occurs.