The main mail server of a financial institution has been compromised at the superuser level; the
only way to ensure the system is secure would be to:

A.
change the root password of the system.

B.
implement multifactor authentication.

C.
rebuild the system from the original installation medium.

D.
disconnect the mail server from the network.

Explanation:

Rebuilding the system from the original installation medium is the only way to ensure all security
vulnerabilities and potential stealth malicious programs have been destroyed. Changing the root
password of the system does not ensure the integrity of the mail server. Implementing multifactor
authentication is an aftermeasure and does not clear existing security threats. Disconnecting the
mail server from the network is an initial step, but does not guarantee security.