Raid configurations of the disks are used to provide
High data density
A & B
Answer is A
RAID stands for Redundant Array of Inexpensive (or sometimes "Independent") Disks. In general, a RAID-enabled system uses two or more hard disks to improve the performance or provide some level of fault tolerance for a machine—typically a NAS or server.
More at : http://in.pcmag.com/storage/65084/feature/raid-levels-explained
Answer is D.
RAID, short for redundant array of independent (originally inexpensive) disks is a disk subsystem that stores your data across multiple disks to either increase the performance or provide fault tolerance to your system (some levels provide both).
You can check here for a more information for RAID.