You want to take a complete database backup using RMAN. The backup should consist only
the used blocks from your database.
Which two statements are true about this backup operation? (Choose two.)
A.
Backup compression should be enabled
B.
Parallelism for the channel should be set to 2
C.
All the files must be backed up as backup sets
D.
The backup may be stored either on disk or on media with media manager
C and D are correct
https://docs.oracle.com/cd/B28359_01/backup.111/b28270/rcmcncpt.htm#i1013399
For A: Backup compression is already enabled-
RMAN compresses the backup set contents before writing them to disk. No extra uncompression steps are required during recovery when you use RMAN compression.
For B: Parallelism only splits the backup process so it runs faster, it doesn’t compress the database.
C: RMAN can store backup data in a logical structure called a backup set, which is the smallest unit of an RMAN backup. A backup set contains the data from one or more datafiles, archived redo logs, or control files or server parameter file. Backup sets, which are only created and accessed through RMAN, are the only form in which RMAN can write backups to media managers such as tape drives and tape libraries.
D: When you execute the BACKUP command in RMAN, you create one or more backup sets or image copies. By default, RMAN creates backup sets regardless of whether the destination is disk or a media manager.