Hi Ranjeet75:
All can be true.
When evaluating questions like this keep in mind that fractional values become smaller when they are squared or quadrupled. The correct answer is all three. Let me give you specific examples.
For I:
Let x = 100, y = 5, z= 1; then y^2 = 25, z^4 = 1 and we have x>y^2>z^4 and x>y>z.
So I can be true.
For II:
Let x = 0.3 ; y = 0.4; z = 0.5 ; then y^2 = 0.16 ; z^4 = 0.0625 and we have x>y^2>z^4 and z>y>x.
So II can be true.
For III:
Let x= 100 ; y = 0.4 ; z = 0.5 ; then y^2 = 0.16 ; z^4 = 0.0625 and we have x>y^2>z^4 and x>z>y
So III can be true.
Hence the final answer is I, II, III