When do you use Hfus? Why can’t we use q=mcΔt at this point?

when there is heat of fusion Hfus we can't use q=mcΔt because there Is a change of state from solid to liquid with out a corresponding change in temperature (hidden heat).

Therefore whenever there is a change from one state to another there is no change in temperature

So instead of using q=mcΔt

We use q=m*Hfus

What is specific Latent heat of fusion?

Specific Latent heat of fusion of a substance is the quantity of heat required to change a kilogram of a substance from the solid state to The liquid state at melting point.

The S. I unit is in joules per kilogram (j/kg)

the electrical  conductivity  of asolution  of an electrolyte is measured by determining the resistance of thesolution  between two flat or cylindrical electrodes separated by a fixed distance.

c8h10 is your answerr