Which of the following is correct when talking about mount points?

The Linux kernel imposes no restriction on using a non-empty directory as a mount point; the only requirement is that the path exists and is a directory. When a filesystem is mounted over a non-empty directory, the original contents are temporarily hidden and inaccessible until the filesystem is unmounted, but they are not harmed.

Using an empty directory is best practice to avoid hiding existing files, but it is not a technical requirement enforced by the kernel or mount command.

The SetUID permission flag has no relationship to mount point eligibility; it is used to execute files with the owner's privileges.

Files inside a directory used as a mount point are not deleted; they are simply hidden while another filesystem is mounted over that directory and reappear after unmounting.