Answer: Gamma Rays
If we divide the electromagnetic spectrum in its different wavelengths in an increasing way, the order is:
Cosmic Rays<Gamma Rays<X Rays< Ultraviolet<Visible Spectrum< Infrared<Microwaves ... and so on
So, if we go from short wavelengths to long wavelengths, those closest to X-rays are gamma rays.
But if we go from long wavelengths to short wavelengths, the closest to X-rays are ultraviolet.
It should be noted that X-rays are ionizing radiation, capable of altering the composition of atoms and body tissues cells. Therefore, it is dangerous and must be handled with care and under strict radiological safety regulations.