A liquid is in equilibrium with its vapor at its boiling point. On the average, the molecules in the two phases have equal

K.E = 3/2kBT, where kB is Boltzmann's constant, is the average kinetic energy's relationship to temperature. The molecules in the two phases have the same kinetic energy when the liquid and its vapor are in equilibrium since they are both at the same temperature.

Therefore, the correct answer is option (C) kinetic energy.