The correct option is A X-rays
To study the structure of the crystal, the wavelength of the wave should be comparable to the size of the crystal and also of high energy.
In the given option Radio wave and infrared waves are not having high energy whereas Gamma rays has high energy but its wavelength is too small, so it will penetrate the structure easily. So, X rays of shorter wavelength are used to study structure of materials because the spacing between the atoms is comparable to its wavelength.