How to recover the data from Redo Log Group which has not been archived yet
If you have access to the logfile in redo group C, and you can copy it to the new server, then you will be able to use it for recovering the database on the new server, so you will not lose the data in it.

If you no longer have access to the redo logfile (because not only the database, but the server or storage also crashed, and you are unable to start/repair them), and all you have is your backup, then yes, you will lose the data created after the last backup.