Which of the following is not a valid LDAP distinguished name (DN)?

A distinguished name (DN) in LDAP (Lightweight Directory Access Protocol) is a unique identifier used to denote an entry in the directory. The DN is structured in a hierarchical format, representing the path from the named entry to the root of the directory.

In the case of the DN presented in the fourth option, the construction is incomplete because it has an equals sign (=) at the end without a corresponding value. A valid DN must complete all components with proper attributes and values. Each component in a DN must include both the attribute type (like cn for common name, uid for user identifier, and dc for domain component) and a corresponding value that follows the equals sign.

For instance, "cn=admin,dc=example,dc=" fails to provide a value after the last equals sign for the domain component, rendering it invalid. In contrast, the other options (with proper attribute-value pairs for each component) conform to the expected structure of a distinguished name.

This understanding highlights the importance of ensuring that each DN is correctly formatted with valid attributes and values, as improper structure, as seen in the fourth option, leads to invalid entries in an LDAP directory.