To solve this question, the user needs to understand the concept of XML elements and attributes, as well as the purpose of ensuring unique identities for these elements and attributes.

Explanation of each option:

A. named pipe: A named pipe is a method of inter-process communication in which two or more processes can communicate with each other by writing and reading from a pipe. This option is unrelated to the concept of unique identities for XML elements and attributes.

B. fully-qualified domain name: A fully-qualified domain name (FQDN) is a domain name that specifies the exact location of a resource on the internet. This option is unrelated to the concept of unique identities for XML elements and attributes.

C. Multiprotocol Label Switching: Multiprotocol Label Switching (MPLS) is a routing technique that directs data packets along predefined paths within a network. This option is unrelated to the concept of unique identities for XML elements and attributes.

D. namespace: A namespace in XML is a mechanism that allows elements and attributes to have unique identities. It ensures that element and attribute names are globally unique, preventing naming conflicts. This is the correct option for ensuring unique identities of XML elements and attributes.

Therefore, the correct answer is:

D. namespace