The correct option is option-1, which states that both the assertion (A) and the reason (R) are individually true, and the reason (R) is the correct explanation of the assertion (A).

In winter, when many lakes and rivers are frozen, marine fish can still survive. This is because the reason states that the density of water is maximum at 4°C. Hence, water below the frozen outer surface is maintained at 4°C. This phenomenon is known as the density anomaly of water. When water cools below 4°C, it starts becoming less dense. This causes the colder water to sink towards the bottom of the lake or river, while the relatively warmer water, which is at 4°C, remains on the top.

This temperature difference allows marine fish to survive even in frozen lakes and rivers. The warm water at 4°C provides a habitat for fish to continue living and thriving during the winter months.

Therefore, the reason (R) correctly explains why marine fish can survive in frozen lakes and rivers during winter, making option-1 the correct answer.