Which of the following is NOT an example of a web application?

HTML is a markup language that is primarily used for structuring and presenting content on the World Wide Web. It provides the foundational framework upon which web applications are built but does not itself constitute an application. Web applications, in contrast, require dynamic interactions and functionalities beyond static content, which HTML alone does not provide.

In the context of the other options, Google Docs, Dropbox, and YouTube are all fully-fledged web applications that allow users to perform specific tasks online, such as document creation and editing, file storage and sharing, and video streaming, respectively. Each of these services involves server-side processing and user interaction, characteristics that define a web application.