A. The organization should write a boot strapping script which will get the MAC address from the instance metadata and use that script to register with the application.
B. The organization should provide a MAC address as a part of the user data. Thus, whenever the instance is booted the script assigns the fixed MAC address to that instance.
C. The instance MAC address never changes. Thus, it is not required to register the MAC address every time.
D. AWS never provides a MAC address to an instance; instead the instance ID is used for identifying the instance for any software registration.