(d) water has highest density of 4°C so underneath the top layer of ice there is layer of water.
Explanation:
In cold countries, when the temperature of fresh water reaches 4°C, the layers of water near the top in contact with cold air continue to lose heat energy and their temperature falls below 4°C. On cooling below 4°C, these layers do not sink but may rise up as fresh water has a maximum density at 4°C. Due to this, the layer of water at 4°C remains at the bottom and above this layers of water 3°C, 2°C, 1°C and 0°C are formed. Because ice is a poor conductor of heat, it does not absorb heat energy from the water beneath the layer of ice which prevents the water freezing. Thus, aquatic creatures survive in such places.
