Switching between portrait and landscape modes involves the Page Setup dialog box (option C).

The Page Setup dialog box is a feature in many software programs, including word processors and graphic design applications. It allows users to customize various aspects of the page, such as margins, orientation, paper size, and more. In the context of switching between portrait and landscape modes, the Page Setup dialog box is used to change the page orientation.

Here is a detailed explanation of the answer:

1. Introduction:
- Switching between portrait and landscape modes involves the Page Setup dialog box.

2. Page Setup dialog box:
- The Page Setup dialog box is a feature in software programs that allows users to customize various aspects of the page.
- It can typically be accessed through the File menu or a toolbar in the program.
- The dialog box provides options for adjusting page settings such as margins, orientation, paper size, and more.

3. Portrait and landscape modes:
- Portrait mode is the default orientation for most documents, where the page is taller than it is wide.
- Landscape mode, on the other hand, is when the page is wider than it is tall.
- Switching between portrait and landscape modes can be useful when creating documents with different layouts, such as brochures, posters, or tables.

4. Changing page orientation:
- To switch between portrait and landscape modes, the user needs to access the Page Setup dialog box.
- In the dialog box, there is typically an option to select the desired page orientation.
- By choosing "Portrait," the page will be set to the default vertical layout.
- By choosing "Landscape," the page will be set to a horizontal layout.

5. Other options in the Page Setup dialog box:
- In addition to changing the page orientation, the Page Setup dialog box may offer other customization options.
- These may include adjusting margins, setting paper size, selecting paper source, adding headers and footers, and more.
- However, for the specific task of switching between portrait and landscape modes, the focus is on the page orientation setting.

In conclusion, switching between portrait and landscape modes involves using the Page Setup dialog box. This feature allows users to customize various page settings, including the page orientation. By accessing the Page Setup dialog box, users can easily switch between portrait and landscape modes to suit their document layout needs.