The reference monitor ensures what?

A.
That only authorized subjects access objects

B.
That information flows from a low security level to a high security level

C.
That the CPU does not access memory directly

D.
That subjects do not write down to lower objects

Explanation:
The reference monitor is an abstract machine that contains the system’s access control security policy. The security kernel enforces the reference monitor and its configurations. Subjects cannot directly access objects without the security kernel and reference monitor allowing it.