Public keys are published through digital certificates, signed by certification authority (CA), binding the certificate to the identity of its bearer.
A bit more details: Although “Digital Certificates” is the best (or least wrong!) in the list of answers presented, for the past decade public keys have been published (ie: made known to the World) by the means of a LDAP server or a key distribution server (ex.: http:// pgp.mit.edu/). An indirect publishing method is through OCSP servers (to validate digital signatures’ CRL)
Reference used for this question: TIPTON, Hal, (ISC)2, Introduction to the CISSP Exam presentation. and http://technet.microsoft.com/en-us/library/dd361898.aspx