A, x, y and z become more nonmetallic. The stronger the nonmetal, the stronger the corresponding oxyacid with the highest valence, but it is not necessarily the highest valence in the options, so A is wrong.
B. nonmetal w (o) > x (p), the stronger the nonmetal element, the weaker the reducibility of the corresponding anion, so b is wrong;
C.x and y are in the same period, and the atomic radius x > y, so c is wrong;
The compound formed by D. O and hydrogen may be H _ H2O2 _ 2, which is nonpolar bond, so D is correct.
So choose D.