The correct answer is D. fuser -mk /data because it directly addresses the issue of a “busy” mount point by identifying and terminating processes that are currently using the filesystem. The error “target is busy” occurs when one or more processes are actively accessing files or directories within the mounted filesystem, preventing it from being safely unmounted.

The fuser command is specifically designed to identify processes that are using files, directories, or sockets. When used with the -m option, it checks all processes accessing the specified mount point. The -k option sends a signal (by default SIGKILL) to terminate those processes. This frees the filesystem, allowing the administrator to successfully run the umount command afterward.

Option A (tree -g /data) is incorrect because it only displays the directory structure and permissions; it does not identify or stop processes using the filesystem.

Option B (ls -ld /data) is incorrect because it shows directory metadata such as permissions and ownership, not active usage.

Option C (stat -f /data) is incorrect because it provides filesystem statistics but does not help identify or resolve active process locks.

From a Linux+ troubleshooting perspective, tools like fuser and lsof are essential for diagnosing resource contention issues. Properly resolving “busy” mount points ensures data integrity and prevents corruption when unmounting filesystems, making this a critical administrative skill.