Thr genotypes of the parents are Hh heterozygote (the father) and hh recessive homozygote (the mother).
The father has a dominant allele (H) that causes him to develop Huntington disease.
As you can see in the punnet square presented below, he will pass this H allele to a half of his offspring, giving them Huntington's disease too.
So, the possibility of the offspring of this couple having Huntington's disease is 50%.
