A user’s Windows 10 workstation with an HDD is running really slowly.

Over time, especially with frequent opening, modification, and saving of large files, a hard disk drive (HDD) can become fragmented. Fragmentation occurs when pieces of individual files are scattered across the disk, leading to longer read and write times, which can significantly slow down the computer. Disk defragmentation reorganizes these fragmented files, placing the pieces closer together on the drive, which improves access time and overall system performance.

Disk Defragment: Using the Disk Defragment tool on a Windows 10 system with an HDD will help in reorganizing the data more efficiently, thus potentially remedying the slow performance issue.

The Registry Editor (B) is a tool that allows for the viewing and editing of the Windows registry and is generally not used for performance improvement but rather for system settings and configuration adjustments. System Information (C) provides a detailed look at the computer's hardware and system details but does not offer tools for performance improvement. Disk Cleanup (D) can help free up space on the hard drive by removing temporary files and system files that are no longer needed, which can indirectly improve performance but is not the primary tool for addressing fragmentation.