The amount of heat needed to melt one mole of a solid at a constant temperature is called
a. molar heat of fusion
c. heat of reaction
b. molar heat of solidification
d. enthalpy

Recall a fusion is the process between liquid and solid. So the energy change between the two is heat of fusion. In a solidification process the heat of fusion is given out, while in a melting process it is given in. Therefore the amount of heat needed to melt one mole of solid at constant temperature is called the molar heat of Fusion.

A.)

I hope this helps.