Which type of encryption is used for the backup performed by using this command?

A?

Sorry, I want to mean B!

B is correct!

To create dual-mode encrypted backup sets, specify the SET ENCRYPTION ON IDENTIFIED BY password command in your RMAN scripts.

To use password encryption, use the SET ENCRYPTION ON IDENTIFIED BY password ONLY command in your RMAN scripts.

BY ON FOR this gives an Syntax error!
probaly they mean:
BY ONLY FOR and than A would be correct!

if:
BY FOR
then B is correct so be Patient and read the question in the real exam carefully!

Making Dual-Mode Encrypted Backups

Use the SET ENCRYPTION BY PASSWORD command at the RMAN prompt to make password-protected backups. If transparent encryption is configured, then omit the ONLY keyword to indicate that the backups are protected with both a password and the configured transparent encryption.

Note:

Create a password that is secure. See Oracle Database Security Guide for more information.

To make dual-mode encrypted backups:

Start RMAN and connect to a target database and recovery catalog (if used).
Execute the SET ENCRYPTION BY PASSWORD command, making sure to omit the ONLY keyword.

The following example sets the encryption password for all tablespaces (where password is a placeholder for the actual password that you enter) in the backup and omits ONLY to indicate dual-mode encryption:

SET ENCRYPTION IDENTIFIED BY password ON FOR ALL TABLESPACES;

Back up the database.

For example, enter the following command:

BACKUP DATABASE PLUS ARCHIVELOG;

B