A hypotonic solution was most likely added to the slide.
The observed change in the cells on the microscope slide, where they appear to swell or expand after the addition of a substance, suggests that a hypotonic solution was introduced. In a hypotonic solution, the concentration of solutes outside the cells is lower than the concentration inside the cells. As a result, water molecules move into the cells by osmosis, causing them to swell and potentially undergo lysis. The increased influx of water leads to the expansion of the cell volume, as depicted in the "After" image.
This characteristic response of cells to a hypotonic environment is crucial in understanding the principles of osmosis and the impact of different solute concentrations on cell morphology. The observed change aligns with the expected cellular behavior in a hypotonic solution, where water moves into the cells, resulting in the noticeable expansion and alteration of cell structure.