An administrator receives the following output while attempting to unmount a filesystem:umount /data1: target is...

Filesystem unmount failures are common troubleshooting scenarios covered in Linux+ V8. When the error “target is busy” appears, it means one or more processes are actively using files or directories within the mount point.

The correct diagnostic command is lsof | grep /data1. The lsof (list open files) utility displays all open files and the processes using them. Filtering the output with grep /data1 identifies exactly which processes are holding file descriptors on the filesystem, preventing it from being unmounted.

The other options are incorrect. ps -f displays process information but does not show open file usage. du -sh calculates disk usage and does not identify active processes. top monitors system performance but cannot pinpoint filesystem locks.

Linux+ V8 documentation emphasizes using lsof or fuser to identify resource locks before unmounting filesystems. Therefore, the correct answer is D.