Assertion (A) : Red blood cells burst when placed in water. Reason (R) : Due to osmosis, water enters into red blood cells.

Assertion (A) is true because red blood cells (RBCs) placed in water, which is a hypotonic solution, will swell and eventually burst in a process known as hemolysis or cytolysis. Reason (R) is also true and provides the correct explanation for the assertion. Osmosis is the net movement of water molecules from an area of high water potential (low solute concentration) to an area of low water potential (high solute concentration) across a semipermeable membrane. Since the interior of an RBC has a higher solute concentration than pure water, water enters the cell to equalize the osmotic pressure. Unlike plant cells, RBCs lack a rigid cell wall to resist this internal pressure, leading to the rupture of the plasma membrane when the influx of water exceeds the cell's volume capacity.

Sources

1. [1] https://www.ncbi.nlm.nih.gov/books/NBK557609/
2. [2] https://www.sciencedirect.com/topics/medicine-and-dentistry/hypotonic-solution