Answer
Infrared is the electromagnetic radiation that has the longest wavelength.
Work Step by Step
a) The wavelength of visible light is 400nm to 700nm
b) The wavelength of ultraviolet light is 10nm to 100nm
c) The wavelength of infrared is 1mm to 100um
d) Lastly, the wavelength of X-rays is 1nm to 10nm
Analyzing the other options, it's clear that infrared electromagnetic radiation has the largest wavelength.