Numerical related to crystallisation of water below its freezing point
In the question Crystallization begins after you add a small piece of ice 
.
 So it has to begin that's been given in the question. 

I don't know thermodynamically whether it's plausible or not, but it's a given in the question a logical alternative way to approach the question (according to me) is


 I formatted it as a spoiler just in case you want to think a bit more or anybody else does



   The temperature has increased because latent heat of fusion is being released i.e ice being formed from water is  releasing heat. 





PS: I may be wrong