Each face of the cube has a diagonal, this diagonal divides one face into two triangle. Looking at face GCDH, it will have diagonal of CH to produce two right angled triangle GCH and DCH.
In triangle GCH, the diagonal CH is the longest side of the triangle which is also the hypotenuse.
Note that the longest side of a right angled triangle is always the hypotenuse.
Therefore triangle GCH is the one that has hypotenuse CH.
