According to the Sitecore XM Cloud Documentation for Developers1, in the Content Editor, you can set placeholder restrictions for your layout placeholders. These restrictions are applied on all pages in the site. For example, if you add restrictions for a footer, these apply for all footers on every page of the site. In this way, you can control the content that content authors can place in layout placeholders.To add a placeholder setting for a site, you need to create a placeholder item in the Layout/Placeholder Settings folder and enter the placeholder key and the allowed components2.

The other options are not correct:

A. The restrictions apply for all placeholders called headless-footer within a single site in the XM Cloud Content Management instance.This option is too narrow, as the placeholder restrictions apply to all sites that use the same placeholder key, not just one site.

C. The restrictions only apply for a placeholder called headless-footer on a designated page.This option is too specific, as the placeholder restrictions apply to all pages that use the same placeholder key, not just one page.However, you can create placeholder restrictions on specific pages in the Experience Editor, if you want to override the default settings3.

D. The restrictions only apply for subsequently created versions of a placeholder called headless-footer.This option is incorrect, as the placeholder restrictions apply to all versions of the placeholder, not just the new ones.

2:Set placeholder restrictions | Sitecore Documentation3:Set placeholder restrictions | Sitecore Documentation1:XM Cloud Documentation for Developers - Sitecore