When a system drive is nearly full, the most appropriate tool to free up space is Disk Cleanup. This built-in Windows utility scans the drive and allows users to safely remove temporary files, system cache, recycle bin contents, and other unnecessary files that consume disk space. Resource Monitor provides detailed real-time system resource usage but does not free space. Disk Defragment reorganizes fragmented files to improve performance but does not delete data. Disk Management is used to partition and format drives but does not clean files. Disk Cleanup is a primary maintenance tool covered in the Core 2 operating system domain to manage storage and optimize system performance.