Answer B is correct. Backup devices can easily be changed from one medium to another with little difficulty. The remaining options are all far more involved than simply redefining the devices.
For more information, see the following sources:
Chapter 6, "Database Maintenance"
SQL Server 2005 Books Online: "SQL Server Database Engine, Administering the Database Engine, Backing Up and Restoring Databases, Managing Backups, Backup Devices"
"sys.backup_devices (Transact-SQL)," http://msdn2.microsoft.com/en-us/library/ms178018(SQL.90).aspx