Which of the following best describes a reference monitor?

A.
An abstract machine that mediates all access subjects have to objects.

B.
A software component that determines if a user is authorized to perform arequested operation

C.
A software component that isolates processes and separates privilege and usermodes

D.
A software component that works in the center protection ring and providesinterfaces between trusted and untrusted objects

Explanation:
A reference monitor is the abstract machine that holds all of the rules of access for the system. The security kernel is the active entity that enforces the reference monitors rules. They control the access attempts of any and all subjects; a user is just one example of a subject.