Correct Option: D

SGML (Standard Generalized Markup Language) is a standard for how to specify a document markup language or tag set. XML and HTML are web markup language used to design and create web pages. The HTML DTD begins with a series of parameter entity definitions. A parameter entity definition defines a kind of macro that may be referenced and expanded elsewhere in the DTD. These macros may not appear in HTML documents, only in the DTD. Other types of macros, called character references, may be used in the text of an HTML document or within attribute values.