Question

why temperature remains constant during boiling even truly the heat is supplied

Answer 1

The temperature at a phase change remains constant because the heat flow in/out of the system is associated with the phase change, not with temperature change. In the case of melting, the heat flowing into the system is called the enthalpy (or heat) of fusion, and in the case of vaporization it is called the enthalpy of vaporization. In both cases the heat absorbed in the phase change is going into separation of the molecules in the more condensed phases.