B (not C): You can validate that all database files and archived redo logs can be
backed up by running a command as follows:
RMAN> BACKUP VALIDATE DATABASE ARCHIVELOG ALL;
This form of the command would check for physical corruption. To check for logical corruption,
RMAN> BACKUP VALIDATE CHECK LOGICAL DATABASE ARCHIVELOG ALL;
D: You can use the VALIDATE keyword of the BACKUP command to do the following:
Check datafiles for physical and logical corruption
Confirm that all database files exist and are in the correct locations.
Note:
You can use the VALIDATE option of the BACKUP command to verify that database files exist
and are in the correct locations (D), and have no physical or logical corruptions that would prevent
RMAN from creating backups of them. When performing a BACKUP...VALIDATE, RMAN reads
the files to be backed up in their entirety, as it would during a real backup. It does not, however,
actually produce any backup sets or image copies (Not A, not E).

Explanation/Reference:
Explanation:
O12c is integrated in database don't have his own agent like in O11G then you cannot start up or shut down a database.

http://docs.oracle.com/cd/E16655_01/server.121/e17639/dp_overview.htm