Question

Which of the following is not diffract x-rays? Solid,liquid,gas,crystal

Answer 1

For the material to diffract x-rays it should posess long-range ordering classically, i.e. it should be a crystal or at least solid. Recently a number of papers on x-ray diffraction of liquids have been published. It occured that short-range ordering is enough to obtain some valuable structural information.
So, the answer is gas - only gas is disordered enough to produce no x-ray diffraction.