What markup language does Extensible Hypertext Markup Language (XHTML) use to organize data on a Web page?

The correct answer is Extensible Markup Language (XML) because XHTML is essentially a stricter and XML-compliant version of HTML. It utilizes XML syntax rules, which means that XHTML adheres to the structure and requirements laid out by XML. This includes features like properly nested tags, the necessity of closing tags, and case sensitivity. By organizing data in this manner, XHTML enhances the interoperability and extensibility of web documents, allowing for better parsing and validation by web browsers and other software.

The other options do not relate directly to the structural foundation of XHTML. Standard Generalized Markup Language (SGML) serves as a parent language from which both HTML and XML were derived, but it is not the language that XHTML uses to organize data specifically on web pages. JavaScript Object Notation (JSON) is primarily a data interchange format used for APIs and not a markup language at all. Cascading Style Sheets (CSS) is a style sheet language used to describe the presentation of a document written in HTML or XML, but it does not organize data; it instead applies styles to markup languages. Thus, focusing on XML's role is key in understanding the organizational structure of XHTML.