The decreasing order of wavelength of infrared, microwave, ultraviolet and gamma rays is:
Infrared, microwave, ultraviolet, gamma rays
Microwave, infrared, ultraviolet, gamma rays
Gamma rays, ultraviolet, infrared, microwaves
Microwaves, gamma rays, infrared, ultraviolet
The wavelength of the Electromagnetic waves is inversely proportional to the frequency of the wave. The frequency of the gamma rays is highest whereas the ultraviolet, Infrared and microwaves follow in decreasing order.
Therefore, the decreasing order of wavelengths will be opposite to the frequency order. So, the microwaves have the longest wavelength and the gamma rays have the smallest.
Microwave > Infrared > Ultraviolet >Gamma ray.