Which Windows command-line tool is used to check and repair file system errors?

Correct answer: chkdsk

Option A is correct because chkdsk (Check Disk) is the Windows command-line utility specifically designed to scan a disk volume for file system errors and bad sectors and, with the appropriate flags, repair them in place. Option B, diskpart, is a disk partitioning tool used to manage disks, partitions, and volumes; it does not scan for or repair file system errors. Option C, sfc /scannow (System File Checker), scans and repairs protected Windows system files, not the file system structure or disk surface errors that chkdsk addresses. Option D, defrag, performs disk defragmentation to optimize file placement for performance and does not check or repair file system integrity.