How is SGML primarily used in web development?

SGML, or Standard Generalized Markup Language, serves as a foundational standard for defining markup languages, making it crucial in the context of web development. It is not a tool or language that is directly used for creating visually appealing web pages, which is what visual design focuses on. While web applications may be built using various programming languages and frameworks, SGML is not specifically designed for that purpose. Additionally, managing server-side processes involves technologies and languages such as server-side scripting, which SGML does not address.

The correct answer highlights SGML's role in establishing guidelines for creating markup languages like HTML and XML. By providing a set of rules for how document structures should be defined, SGML enables developers to create custom markup languages tailored to specific needs, which are utilized across different domains of computing and documentation. Thus, C effectively captures the essence of SGML's utility in web development.