The difference between a security kernel and a reference monitor is that:

a. A security kernel is a component implementation of a security policy, whereas a reference monitor is an abstract representation of a security policy

b. A reference monitor is the component in a security kernel that handles access to the resources to be protected

c. A reference monitor is used to protect resources in a single-state machine, whereas a security kernel is used to protect resources in a multiple-state machine

d. A reference monitor enforces a discretionary security policy, whereas a security kernel enforces a mandatory security policy