Final answer:
In the Kerberos protocol, the permanent keys are Key A and Key B, while Key C is a session key. So, the correct option is Key AB.
Step-by-step explanation:
In the Kerberos protocol, the keys that are considered permanent are Key A and Key B. These keys are long-term shared secret keys that are established between the Kerberos authentication server and the client.
Key C, on the other hand, is a session key that is generated for each communication session between the client and the server. It is not considered permanent.
Therefore, the correct option from the given choices would be Key AB.