The heat of fusion of a substance is the energy measured during a 1. phase change 2. temperature change 3. chemical change 4. pressure change

option 1, phase change.

Explanation:

The phase change is physical change that consists in the pass of a substance from one state to another.

The main phase changes are:

1. Fusion or melting: change from solid to liquid

2. Freezing: the reverse to fusion or melting, i.e. the change from liquid to solid

3. Boiling:  change from liquid to gas

4. Condensing: the reverse to boiling, i.e. the change from gas to liquid

5. Sublimation: the pass from solid to gas, without passing through liquid state.

6. Deposition: the reverse to sublimation, i.e. the change from gas to solid, without passing through liquid.

During all the phase changes, the energy supplied to the system or released by the system is used to modify the space between the particles that constitute the substance, and so the temperature of the substance does not change.

The heat of fusion, also called latent fusion heat, is the energy measured during the fusion or melting process, this is the energy absorbed by the substance when it is transformed from solid state to liquid, which occurs at constant temperature, and constant pressure.

The absorbed energy is used to overcome the large intermolecular forces that keeps the particles tightly packed in the solid state, increasing the energy of the particles and causing them to separate (occupy more space) in the liquid state. So, fusion is an endothermic process.