The E.M. wave with shortest wavelength among the following is

If we look at the spectrum of electromagnetic waves then the order of a wavelength is, ${\lambda }_{gamma}<{\lambda }_{X-ray}<{\lambda }_{UV rays}<{\lambda }_{Visible}<{\lambda }_{Infrared}<{\lambda }_{Microwave}<{\lambda }_{Radio}$, so from the above spectrum the wavelength of gamma rays is minimum and then the wavelength of a radio wave is maximum, so the shortest wavelength is for gamma-rays.