When human red blood cells are placed in pure water, the cells immediately burst. which process explains what happens to the red blood cells?

The process of osmosis explains what happened to the cells.

Osmosis is where water molecules will flow from a region of higher water potential to a region of lower water potential, through a partially permeable membrane. Red blood cell is a human cell and have a partially permeable membrane. At the same time, pure water has the highest water potential among all substances, and of course higher than the cytoplasm of the red blood cell, therefore, water molecules will flow from the water to the red blood cell.

So much water has flown to the red blood cells that the cell membrane cannot hold all and eventually the cell bursts. Therefore, the process of osmosis explains this result.