Which of the following is a step for customizing margins in a document?

Customizing margins in a document often involves adjusting the settings to meet specific formatting requirements. Selecting "Custom Margins" under the Page Layout tab provides direct access to the margin settings where you can specify measurements for the top, bottom, left, and right margins. This option allows for precise control over how much space is allocated around the text on the page, which is essential for achieving a polished and professional-looking document.

The other options do not pertain to the process of customizing margins. Printing (the first option) does not involve margin adjustments and is a separate function altogether. Inserting a header (the third option) is about adding a title or information at the top of each page, which does not affect the margin settings directly. Changing page orientation to landscape (the fourth option) alters the layout of the page but does not specify or customize the margin measurements themselves. Therefore, selecting Custom Margins is the clear and correct step for modifying margins in a document.