When the Android device appears offline while trying to debug

I struggled a bit with my Android device appearing offline while trying to deploy an app I was working on to my phone. I was trying running adb devices to figure out if the system could communicate with the phone.

./adb devices
List of devices attached
<serial_number> offline 

At least since version 4.2.2, there is a security question that appears on the phone requesting RSA verification with the computer. Be sure your tools are updated AND you allow the PC access by verifying the security question on the devices in question. This fixed it for me. And as always, verify you have debugging enabled in the developer options ;)