Correct Answer: A
This is often implemented by a one-for-one disk-to-disk ratio.
RAID Level 2 provides redundancy by writing all data to two or more drives set. The performance of a level 1 array tends to be faster on reads and slower on writes compared to a single drive, but if either of the drive sets fails, no data is lost. This is a good entry-level redundant system, since only two drives are required as a minimum; however, since one drive is used to store a duplicate of the data, the cost per megabyte is high. This level is commonly referred to as mirroring.
Please visit http://www.sohoconsult.ch/raid/raid1.html for a nice overview of RAID Levels.
For the purpose of the exam you must be familiar with RAID 0 to 5, 10, and 50.
References:
http://www.sohoconsult.ch/raid/raid1.html
and
KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten
Domains of Computer Security, 2001, John Wiley & Sons, Page 65.