Answer: -
Kinetic energy.
Explanation: -
Solids have the least kinetic energy and the highest intermolecular force of attraction among the three states of matter.
When heat energy is supplied to the solid, the kinetic energy of the solid particles increases. At some point, the kinetic energy becomes comparable to the intermolecular force of attraction. At that point solids change into liquids.
When further heat energy is given, at certain point the kinetic energy becomes far greater than the intermolecular forces of attraction. At that point it becomes gas.