The correct answer is B. Site Manager.
Nvu is a web authoring tool that allows you to create and edit websites. Among its various features, the Site Manager tool is specifically designed to help you navigate through your site easily.
The Site Manager provides a user-friendly interface that allows you to organize and manage the files and directories of your website. It allows you to view the structure of your site, create new pages or directories, and edit existing ones. With the Site Manager, you can easily access different sections of your website and make changes as needed.
While the other options mentioned (A. CSS editor, C. Markup cleaner, and D. Validate HTML) are also important tools in Nvu, they do not specifically aid in navigating through your site. The CSS editor is used for editing cascading style sheets, the Markup cleaner helps in cleaning up and optimizing HTML code, and the Validate HTML tool checks the validity of your HTML code.