Nickie is responsible for cryptographic keys in his company. What is the best way to deauthorize a public key?

Certificate revocation lists (CRLs) are designed specifically for revoking certificates. Since public keys are distributed via certificates, this is the most effective way to deauthorize a public key.
Sending out a network alert is incorrect. Simply notifying users that a key/certificate is no longer valid is not effective.
Delete the digital certificate is incorrect. Deleting a certificate is not always possible and ignores the possibility of a duplicate of that certificate existing.
Option Notify the RA is incorrect. The registration authority (RA) is used in creating new certificates, not in revoking them.