The C option replaces the W option, and performs the same task that W used to perform—it wipes out the hard disks.
The C option is useful if you have 7825/28-H3 servers running with a Release 8.6 (or higher) version of SWRAID. If so, you can perform a fresh install of any version of pre-8.6 Unified Communications Manager by booting your systems with the 8.6 (or higher) version of the Recovery Disk. You must then select option C to clear/wipe out the system.
For information about running the Recovery Disk application on your virtual machine, see the Obtain and Run Recovery Software on the CUCM VM tech note at http://www.cisco.com/c/en/us/support/docs/unified-communications/unified-communications-manager-callmanager/118948-technote-cucm-00.html