The portion of the electromagnetic spectrum that plays a role in the activation of vitamin d is visible light ultraviolet light infrared radiation microwaves

The answer is Ultraviolet Light

The easiest way for us to obtain a sustainable amount of Vitamin D is by exposing our self to morning sunlight ( which contain ultraviolet light)

Often time, we don't consume enough vitamin D and our body need to get it from our bones, and in order to do that, it need to be activated through enough exposure from sunlght