When dissolved in water, NaOH flakes show an exothermic change. This means that heat is released during the dissolution process. The reason for this is that the interaction between NaOH and water molecules releases energy in the form of heat. This can be easily observed by touching the container in which the dissolution is taking place. It will feel warm to the touch due to the energy being released. Therefore, the correct option is "An exothermic change."