Why would a certificate authority revoke a certificate?

A.
If the user�s public key has become compromised

B.
If the user changed over to using the PEM model that uses a web of trust

C.
If the user�s private key has become compromised

D.
If the user moved to a new location

Explanation:
C: The reason a certificate is revoked is to warn others who use that persons public key that they should no longer trust the public key because, for some reason, that public key is no longer bound to that particular individuals identity. This could be because an employee left the company, or changed his name and needed a new certificate, but most likely it is because the persons private key was compromised.