Explanation
A hard link is a directory entry that refers to the same inode as another file. An inode is a data structure that stores the metadata and the location of the data blocks of a file. A hard link allows multiple names to refer to the same file content, as long as they are on the same filesystem. However, if the source and the target of the hard link are on different filesystems, the ln command will return an error, because the inode numbers are not consistent across different filesystems. Therefore, the ln command cannot create a hard link that crosses filesystem boundaries. The other options are either incorrect or not applicable. The source file being hidden, read-only, a shell script, or not owned by the user does not prevent the creation of a hard link, as long as the user has the permission to write to the target directory and the source and the target are on the same filesystem. References:
* LPIC-1 Exam 101 Objectives, Topic 103: GNU and Unix Commands, 103.3 Perform basic file management
* LPIC-1 Linux Administrator 101-500 Exam FAQ, LPIC-1 Exam 101 Objectives, GNU and Unix Commands (Total Weight: 25)