In which region is the substance in both the solid phase and the liquid phase ?

Region 3

Explanation:

When there is a continual increase in temperature, a substance undergoes change of state. From the diagram, the substance would be a complete solid in region 1 because of the very low temperature. At region 2, the temperature has increased but it still retain its initial state. But at region 3, the substance start to melt as the temperature increases, thus existing as solid and liquid.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.

answerlinnaeus;