Final answer:
The transition from the solid state to the liquid state of a pure substance is characterized by the heat of C) fusion, which is the energy required to melt the solid without changing its temperature.
Step-by-step explanation:
The transition between the solid state and the liquid state of a pure substance is characterized by the heat of fusion. This is the amount of energy required to change a substance from the solid phase to the liquid phase without changing its temperature.
This process is known as melting or fusion, and the necessary energy for this phase change is referred to as the enthalpy of fusion (AHfus).
While the heat of sublimation refers to the transition from a solid directly to a gas, and the heat of vaporization concerns the transition from liquid to gas, the question specifically asks about the solid to liquid transition. Therefore, the correct answer here is C) Fusion.