The correct answer is it would swell and then burst.
A hypotonic solution is a solution that has a lower concentration of salts and other molecules than the one that is present inside of the cell. Since the cell membrane that surrounds the cell's cytoplasm is permeable for water, if an unicellular organism was exposed to a hypotonic solution due to the process of osmosis, the water molecules will start to enter the cell to even out the concentrations of the two solutions. If the cell remained in this position over a long period of time it would swell and ultimately burst. Organisms and cells developed various ways of protecting from this, like cell walls, vacuoles, special molecules that elevate the concentration of the cytoplasm etc.
