The enthalapy of vaporization of water is given as 2257 KJ/kg. Consequently, enthalapy of condensation will be given as -2257 KJ/kg
During condensation, this much amount of energy would be released for complete condensation of water.
The given mass is 15 gm = 0.015 kg
Thus, the energy needed for condensation is 0.015 kg x -2257 KJ/kg = -33.855 KJ.
Thus, 33.855 KJ of energy would be released for condensation of 15 gm of water vapors.