Question

suppose you are designing a device to transmit information without wires what part of the EM spectrum will your device use and why

Answer 1

My device will use either radio waves or light waves.
Those are the only EM waves I'm equipped to generate.

If I want to get elegant about it, I may use radio waves with
frequencies around 14.1 MHz, because those can be received
many thousands of miles away.  I know how to build an effective
antenna to radiate at that frequency, and as a licensed ham-radio
operator, I can do it legally.