A systems administrator is unable to back up user files. Which of the following is the BEST explanation for this issue?

When a file is open, the OS places an exclusive lock on it that prevents other processes - including backup software - from reading or copying it. Standard file-based backup agents cannot access locked files and will skip or fail on them. Technologies like Volume Shadow Copy Service (VSS) exist specifically to work around this limitation by creating a point-in-time snapshot.

Files that have already been copied retain no lock or restriction, so they remain fully accessible to backup software.

Modified files are simply flagged as changed for incremental or differential backups and are still accessible - modification status does not block backup access.

Compression is a file attribute that does not restrict read access, so compressed files can be backed up without issue.