There are multiple devices that emit radio waves. The most obvious is are radiod and microwaves (yes, the food cooker). Wireless applicances also emit radio waves. But don't confuse everything to use microwaves. Some applicances use frequencies instead of radio waves. Some applicances that use frequencies are toy control remotes and a garage door.
I hope this helps! Please make sure ask me if you need more clarity!
It could be c or d because when you are chatting to someone through say Skype you use fonts and colors to identify who you are talking to. But in most cases it would be d