Osmosis is responsible for plasmolysis. Osmosis is a type of diffusion in which water flows into or out if the cell through plasma membrane.

A solution is a mixture of fluid, solute and solvent. Hypertonic is a solution in which there is high concentration of solutes outside the cell. So, the water flows out of the cell to balance the concentration both sides. Cell will shrink.

Hypotonic solution has the concentration of solute higher outside therfore it results movement inside cell. It leads to swelling up of cell.

Isotonic solution have equal concentration outside and inside of cells.

B. If a plant cell placed in a solution having high water potential, then it has more solutes.  Therefore it is a hypertonic solution.