Which command is used to check disk integrity and repair errors in Windows?

Correct answer: chkdsk

Option C is correct because `chkdsk` (Check Disk) is the Windows command-line utility that scans a volume for file system errors and bad sectors, and can repair logical file system errors when run with the appropriate flags such as `/f` and `/r`. Option A is incorrect because `diskpart` is a disk partitioning and management tool used to create, delete, and format partitions, not to check or repair file system integrity. Option B is incorrect because `scandisk` was a disk-checking utility from older Windows versions (Windows 9x era) and is not available in modern Windows operating systems. Option D is incorrect because `defrag` is the disk defragmentation utility, which reorganizes fragmented data for performance but does not scan for or repair file system errors.