Which option is essential for customizing a document's layout in Word?

Customizing a document's layout in Word is primarily achieved through the Page Setup feature. This option allows you to define essential layout elements such as margins, orientation (portrait or landscape), and page size. For example, adjusting margins can significantly impact the overall visual appearance of the document, allowing for more space or more content on a page. Setting the page orientation can help fit specific types of content better, such as wider tables or images.

The other options, while useful in other contexts, do not directly influence the overall layout of the document itself. Content Controls are more focused on facilitating user interactivity within forms or templates. Text Effects apply stylistic choices to text but do not change the document’s structural layout. The References Tool is used for managing citations and bibliographies rather than altering the physical layout of the pages. Thus, Page Setup is the key option for any adjustments related to the layout of a document.