Implication chart method for state reduction
Let's first look at the a-b box (the topmost one). In the box it says d-f, c-h. First you look at the d-f box, there is a cross. This means that d-f is invalid. Thus any box that contains d-f, in this case it will be the a-b box which we are concerned right now, should be crossed out. Repeat it again to other "alive box"s.