Question: XHTML modules can be extended by adding elements, attributes, modifying content models, or some combination of these. What does a proper implementation of an XHTML module require?

1. The implementation of an XHTML module requires an extension module and a validation module that ensures that the XHTML is well formed and valid; otherwise the extended instances aren't formally XHTML.
2. The implementation of an XHTML module requires a definitions module and a constraint module that specifies syntax rules and uses the parameter entities declared in the definitions module.
3. The implementation of an XHTML module requires a qualified name module and a declaration module that holds the element, element attribute, and content model declarations.
4. The implementation of an XHTML module requires a namespace module that holds the element, element attribute, and content model declarations, and a parameter module that uses the entities declared in the namespace module.

Answer: The correct answer of the above question is Option D:The implementation of an XHTML module requires a namespace module that holds the element, element attribute, and content model declarations, and a parameter module that uses the entities declared in the namespace module.