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) lack a rigid cell wall, making them susceptible to bursting when placed in a hypotonic environment like pure water [t6, t9]. Reason (R) is also true and provides the correct explanation: water is a hypotonic solution relative to the cell's cytoplasm, which contains higher solute concentrations [t3, t9]. Through the process of osmosis, water molecules move across the semi-permeable plasma membrane into the cell to equalize concentration [t3, t6]. This influx causes the RBC to swell until it reaches its critical hemolytic volume, at which point the membrane ruptures, a process known as hemolysis [t2, t5, t8]. Unlike plant cells, which have a cell wall to resist such osmotic pressure, animal cells like RBCs have limited expansion capacity and will lyse under these conditions [t6, t9].