When a red blood cell is placed in pure water, it will swell and eventually burst. This occurs because of the process known as osmosis.
Osmosis is the movement of water across a semi-permeable membrane (like the membrane of a red blood cell) from an area of lower solute concentration (pure water) to an area of higher solute concentration (inside the red blood cell). Since the inside of the red blood cell has a higher concentration of solutes compared to pure water, water rushes into the cell to equalize the concentrations.
Initially, the cell will swell as water enters, but if enough water makes its way in, the cell will eventually burst, a phenomenon known as hemolysis. Therefore, the correct answer is (b) swell and burst.