Your published layout.cshtml file contains a static placeholder with a “main” placeholder key and you want to add renderings to the presentation details of a templates standard values; however you cannot see the "main" placeholder key you added to the layout. Of the available options, which is the best explanation of what happened?
When adding a static placeholder to a layout.cshtml file, it's necessary to create a corresponding placeholder settings item in the content tree. This item defines the allowed placeholder keys and is associated with the placeholder key used in the layout. If the "main" placeholder key added to the layout is not appearing when editing the presentation details of a template's standard values, it indicates that there is no corresponding placeholder settings item for the "main" placeholder key.
I think the answer is D https://sitecorediaries.org/2019/12/09/sitecore-placeholders-and-placeholder-settings/
It's C, you modify the presentation details, you need to publish
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.