Best way to approach proximity sensoring

Hi guys, new here!

I am working on a project where I need to figure out if Person A. is away from Object B. Possibly by a distance of 3-5 feet. And must be able to work through a door.

I have been searching for sensors that I could use say one arduino stationary and maybe the person is holding a chip/receiver and when that person gets far enough away from the main emitter I can have an alarm set or a notification triggered.

Any ideas would definitely be helpful

Ive research RF, IR, UltraSonic sensors having a tough time figuring out which will work.

Thanks