WHY WATER REACHES MAXIMUM DENSITY AT 3.98 DEGREE CENTIGRADE?

It is due to strong molecular interactions of the hydrogen bonds as the water is cooled. Most materials become more dense as they cool, and so does water but when it gets close to freezing its density is maximum and it gets less dense and at freezing (0 C) it expands and becomes ice, which floats. Not many materials do this. The 4C max density number is a property of the water