In Sitecore, when you add a static placeholder to your layout file, you typically need to create a corresponding placeholder settings item in the content tree. This placeholder settings item defines the allowed placeholder keys and is associated with the placeholder key used in your layout. If you have added a "main" placeholder key to your layout.cshtml file, but it is not appearing when you edit the presentation details of a template's standard values, it's likely that there is no corresponding placeholder settings item for the "main" placeholder key. To resolve this, you should create a placeholder settings item in the content tree with the key "main" and associate it with the appropriate allowed controls/renderings.

It's C, you modify the presentation details, you need to publish

I think the answer is D https://sitecorediaries.org/2019/12/09/sitecore-placeholders-and-placeholder-settings/