Answer: The correct answer is "type of medium".
Step-by-step explanation:
Electromagnetic wave is the wave in which both electric field and the magnetic field are mutually perpendicular each other to the motion of the wave. It travels with the speed of light.
When the electromagnetic wave travels from one medium to the another medium then the speed of the wave will get change and therefore, it changes the wavelength.
Therefore, the type of medium is responsible for altering the speed of an electromagnetic wave.