You have to use the latent heat of vaporization of the water.
The latent heat of vaporization of water at 100 °C is approximately 2.26 kJ / g (search this number in some tables; you will find similar numbers aroung 2.23 and 2.26 kJ/g).
Now mutiply 25.0 g * 2.26 kJ / g and you will get 56.5 kJ
The answer is approximately 56.5 kJ.
