Which of the following are considered metalanguages?

Metalanguages are languages that are used to create or describe other languages. In the context of web development and markup languages, both Standard Generalized Markup Language (SGML) and Extensible Markup Language (XML) serve as foundational structures that define how text and data should be organized and processed.

SGML is a standard for defining generalized markup languages for documents. It provides a framework for markup languages, which can express a variety of data structures. XML, which is derived from SGML, allows users to define their custom tags and the structure of the data, making it extensible and adaptable for various applications. Both SGML and XML enable the creation of structured documents, which adhere to specific syntax rules, and thus qualify as metalanguages.

The other options involve languages or formats that do not fit the definition of a metalanguage. HTML and CSS are markup and style sheet languages, respectively, used for presenting content on the web, while PHP is a server-side scripting language and SQL is a language for database manipulation. JSON and CSV serve as data interchange formats or text-based structures for data representation, but they do not define the structure of other languages. Therefore, SGML and XML are the clear examples of metalanguages because of their roles in defining