a. light
Light waves do not require a physical medium through which to travel. They can travel through a vacuum, such as in outer space. They are also considered as electromagnetic waves, which means they are oscillating electric and magnetic fields that travel through space as waves.
The answer to the first question is: d. electric only
Light waves are a form of electromagnetic waves, which are oscillating electric and magnetic fields that travel through space as waves. They do not have a sound-like component.