Enthalpy of Fusion
The enthalpy of fusion is a thermodynamic property that represents the amount of energy required to change a substance from a solid to a liquid phase at a constant temperature and pressure. It is also known as the heat of fusion.

Significance of Enthalpy of Fusion
The enthalpy of fusion is an important property as it quantifies the energy required to overcome the intermolecular forces holding the solid together and convert it into a liquid. This energy is used to break the attractive forces between the molecules in the solid and allow them to move more freely in the liquid phase.

Positive Enthalpies of Fusion
The enthalpies of fusion are always positive. This means that energy is required to break the intermolecular forces and convert the solid into a liquid. The positive sign indicates that the process is endothermic, meaning it absorbs heat from the surroundings.

Explanation
The positive sign of the enthalpy of fusion can be understood by considering the nature of the intermolecular forces in the solid state. In a solid, the molecules are closely packed and held together by strong intermolecular forces such as hydrogen bonding, dipole-dipole interactions, or London dispersion forces.

To convert the solid into a liquid, these intermolecular forces must be overcome. This requires the input of energy, which is reflected in the positive value of the enthalpy of fusion. The energy is used to break the attractive forces and allow the molecules to move more freely in the liquid phase.

Additionally, the positive sign of the enthalpy of fusion is consistent with the general trend of enthalpy changes. In most processes, including phase changes, energy is required to break existing bonds or forces and form new ones. This energy input is reflected in the positive sign of the enthalpy change.

Therefore, it can be concluded that all enthalpies of fusion are positive, indicating that energy is required for the solid to liquid phase transition. The correct answer is option 'A' - true.