Remember the term Public-Key Certificate is synonymous with Digital Certificate or Identity certificate.
The certificate itself provides the binding but it is the certificate authority who will go through the Certificate Practice Statements (CPS) actually validating the bindings and vouch for the identity of the owner of the key within the certificate.
As explained in Wikipedia:
In cryptography, a public key certificate (also known as a digital certificate or identity certificate) is an electronic document which uses a digital signature to bind together a public key with an identity
- information such as the name of a person or an organization, their address, and so forth. The certificate can be used to verify that a public key belongs to an individual.
In a typical public key infrastructure (PKI) scheme, the signature will be of a certificate authority (CA). In a web of trust scheme such as PGP or GPG, the signature is of either the user (a self-signed certificate) or other users ("endorsements") by getting people to sign each other keys. In either case, the signatures on a certificate are attestations by the certificate signer that the identity information and the public key belong together. RFC 2828 defines the certification authority (CA) as:
An entity that issues digital certificates (especially X.509 certificates) and vouches for the binding between the data items in a certificate.
An authority trusted by one or more users to create and assign certificates. Optionally, the certification authority may create the user's keys.
X509 Certificate users depend on the validity of information provided by a certificate. Thus, a CA should be someone that certificate users trust, and usually holds an official position created and granted power by a government, a corporation, or some other organization. A CA is responsible for managing the life cycle of certificates and, depending on the type of certificate and the CPS that applies, may be responsible for the life cycle of key pairs associated with the certificates
Source: SHIREY, Robert W., RFC2828: Internet Security Glossary, may 2000.
and http://en.wikipedia.org/wiki/Public_key_certificate

The correct answer is "Legitimate users accessing networked services that would normally be restricted to them". Logon abuse entails an otherwise proper user attempting to access areas of the network that are deemed offlimits. Answer "Breaking into a network primarily from an external source" is called network intrusion, and d refers to backdoor remote access.