Active Directory is correct because it is a Microsoft product and therefore a proprietary implementation rather than an open standard.

Active Directory builds on and interoperates with open protocols such as LDAP and Kerberos but Microsoft defines product specific extensions, the AD schema and the replication and management features. Those vendor controlled extensions and the product packaging are what make it a proprietary directory and identity implementation.

Kerberos is not correct because it is an open authentication protocol that originated at MIT and is standardized in RFC 4120 and it is implemented by many vendors and platforms.

IEEE 802.1X is not correct because it is an IEEE standard for port based network access control and it is an open standard used for network authentication.

X.500 is not correct because it is an ITU and ISO directory standard that defines an open directory model and protocols and it was the basis for LDAP.

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