Specific latent heat of fusion is defined as:

Specific latent heat of fusion is defined as the energy required to melt 1 kg of a substance at its melting point, without changing its temperature. This process involves breaking the bonds between the particles in a solid, allowing them to move freely as a liquid. The energy absorbed during this phase change does not raise the temperature of the substance but is instead used to alter its state from solid to liquid.

In contrast, other choices describe different processes. For instance, the energy required to vaporize 1 kg of liquid pertains to the specific latent heat of vaporization, which deals with changing a liquid to a gas. Cooling a substance or increasing the temperature of a solid relates to changing thermal energy and temperature, rather than a change in state. Thus, the definition of specific latent heat of fusion is specifically tied to the phase change from solid to liquid, making the first choice the correct answer.