Assertion (A) : In winter, although many lakes and rivers are frozen, the marine fish still survives. Reason (R) : The density of water is maximum at 4°C. Hence, water below the frozen outer surface is maintained at 4°C.

The assertion is true as marine and freshwater fish survive in frozen lakes and rivers due to the anomalous expansion of water. Water reaches its maximum density at 4°C [t3][t8]. As surface water cools to 4°C, it sinks, displacing warmer water until the entire body reaches this temperature [t6][t8]. Further cooling below 4°C makes the surface water less dense, causing it to remain on top and eventually freeze at 0°C [t1][t5]. This floating ice layer acts as an insulator, preventing the deeper water from freezing and maintaining it at a stable 4°C [t8][t9]. This thermal stratification ensures a liquid habitat remains available for aquatic life throughout winter [t5][t10]. Since the density property directly explains why the bottom remains liquid and habitable, the reason correctly explains the assertion.